An attack at the US Capitol complex in Washington DC has left one police officer dead and another in hospital with injuries.
A car crashed into a security barrier before the driver lunged towards the officers with a knife, police said.
The officers opened fire and the suspect was shot dead.
Authorities said the attack, which came three months after January's deadly Capitol riots, did not appear to be terrorism-related.
"Whether the attack was at law enforcement, or whoever, we have a responsibility to get to the bottom of it and we'll do that," Robert Contee, the acting chief of Washington DC's Metropolitan Police Department, said at a news conference.
Acting Capitol Police Chief Yogananda Pittman said it was "with a very, very heavy heart that I announce one of our officers has succumbed to his injuries".
In a later statement, she named the officer as William "Billy" Evans, who had been a member of the Capitol Police for 18 years and was part of the department's first responders unit.
"Please keep Officer Evans and his family in your thoughts and prayers," she said.
Two law enforcement sources involved in the investigation told BBC partner CBS News that the suspect in the attack was 25-year-old Noah Greene from Indiana.
They said that no prior information about him had been found on any police databases.
An increased security presence remains around the Capitol building, where the US Congress sits.
Congress is currently in recess, meaning the majority of politicians were not at the Capitol complex at the time of the attack.
What do we know about the attack?
Shortly after 13:00 local time (18:00 GMT) the Capitol Police alert system sent an email to lawmakers and their staff ordering them to stay away from exterior windows and doors due to a threat. Anyone outside was instructed to seek cover.
At that time, a man driving a blue sedan had rammed the car into two officers standing at the North Barricade, according to police.
He exited the vehicle and ran towards the officers - at least one of whom drew a weapon and shot the suspect. The officers were then transported to hospital, one in an ambulance and one in a police cruiser.
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Footage of the scene showed a helicopter flying overhead and what appeared to be two people on stretchers being moved into ambulances.
Onlookers were told to clear the area.
Police said the suspect died due to his injuries at 13:30 local time. Chief Contee told reporters the suspect appeared to act alone.
The FBI's Washington Field Office said it was providing support to the Capitol Police.
A second attack in less than three months
Analysis by Samantha Granville, US Capitol
Capitol Hill has been tense over the past three months. Since the riot on 6 January, the complex has been like a fortress with barbed wire, metal fencing and heightened security.
But just weeks later, we are back here with blocked roads, extra troops, and a solemn feeling.
Congress is in recess today and staff I have spoken to are grateful to be home and nervous about returning to work after the Easter holiday.
They say, understandably, that it is scary to have your workplace attacked twice in a short space of time.
It is concerning for them that even with ramped up security, an event that led to an officer's death still happened.
What has the reaction been?
US President Joe Biden said he was "heartbroken to learn of the violent attack".
He offered his "heartfelt condolences to Officer Evans' family, and everyone grieving his loss", and said that he had ordered flags at the White House to be lowered to half mast.
Democratic House speaker Nancy Pelosi said "America's heart has been broken" and called Mr Evans "a martyr for our democracy".
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Democratic Senate majority leader Chuck Schumer said he was "heartbroken for the officer killed" and indebted to the Capitol police force.
Republican Senate minority leader Mitch McConnell wrote that he was "praying" for those attacked.
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Russia has warned Nato against sending any troops to help Ukraine, amid reports of a large Russian military build-up on its borders.
Kremlin spokesman Dmitry Peskov said Russia would take "additional measures" if Nato were to make such a move.
Sporadic, low-level clashes continue in eastern Ukraine between Ukrainian troops and Russian-backed separatists.
US forces in Europe are now on higher alert, citing "escalations of Russian aggression" in the area.
A Nato official told Reuters news agency that Russia was undermining efforts to reduce tensions in eastern Ukraine and Nato ambassadors had met on Thursday to discuss the situation.
"Allies share their concerns about Russia's recent large-scale military activities in and around Ukraine," the official said.
Ukraine's President Volodymyr Zelensky joined the criticism, saying "military exercises and possible provocations along the border are traditional Russian games".
In a phone call with Mr Zelensky on Friday, US President Joe Biden "affirmed the United States' unwavering support for Ukraine's sovereignty and territorial integrity in the face of Russia's ongoing aggression in the Donbas and Crimea", the White House said in a statement.
Meanwhile, Mr Peskov accused Ukrainian forces of staging "provocations" on the frontline in eastern Ukraine, where a fragile ceasefire is violated daily.
Ukraine's Foreign Minister Dmytro Kuleba told the BBC that there had been a build-up of Russian military at the northern border between Ukraine and Russia, along the eastern border and "also in the illegally occupied Crimea".
"So from three directions we observed a military build-up of the Russian federation," he said, adding: "Ukraine is not looking for any escalation - we do not need war."
In the worst flare-up in recent months, four Ukrainian soldiers died in separatist shelling on 26 March near Shuma, a village in the Donetsk region. There have been only low-level incidents since then.
What is the scale of Russian involvement?
Last month Russia conducted military exercises in Crimea, the Black Sea peninsula it annexed from Ukraine in March 2014.
The rebels in the Donbas region of eastern Ukraine also rely on Russian help. Western governments say Russia has deployed regular troops there, along with heavy weapons. That is denied by the Kremlin, which says Russian "volunteers" are helping the rebels, who seized a swathe of the Donetsk and Luhansk regions in April 2014.
Unverified video on Twitter in recent days purports to show Russian tanks, artillery and armoured vehicles heading to the Ukrainian border.
Ukraine's army commander Gen Ruslan Khomchak says Russia has deployed 28 battalion tactical groups near Ukraine's eastern border and in Crimea, which would amount to 20,000-25,000 troops. Russian officials have not confirmed that, nor given any precise figures.
According to Gen Khomchak, Russia also has nearly 3,000 officers and military instructors in the rebel units in eastern Ukraine.
"The Russian Federation moves its armed forces within its territory at its discretion," Kremlin spokesman Mr Peskov said. He added that "it should not worry anyone and does not pose a threat to anyone".
The Russian military has confirmed that an airborne assault brigade - about 4,000 troops - is being redeployed to Crimea from Volgograd in southern Russia this year.
Since 2014 Russia has considerably beefed up its military presence in Crimea, including its key Sevastopol naval base.
An attack at the US Capitol complex in Washington DC has left one police officer dead and another in hospital with injuries.
A car crashed into a security barrier before the driver lunged towards the officers with a knife, police said.
The officers opened fire and the suspect, who has not been identified, was shot dead.
The city's acting police chief has said the attack does not appear to be terrorism-related.
"Whether the attack was at law enforcement, or whoever, we have a responsibility to get to the bottom of it and we'll do that," Robert Contee, the acting chief of Washington DC's Metropolitan Police Department, said at a news conference on Friday.
"It is with a very, very heavy heart that I announce one of our officers has succumbed to his injuries," Acting Capitol Police Chief Yogananda Pittman said at the conference.
The exact circumstances of the officer's death are not yet clear.
There is an increased security presence around the Capitol building, where the US Congress sits, and a number of police cars surrounding the area.
President Joe Biden left Washington earlier in the day for Camp David, the presidential retreat in Maryland. The White House said the president has been briefed on the situation.
However, some reporters, maintenance workers and Capitol Hill employees are likely still on the Capitol grounds.
The lockdown order on the Capitol was lifted just after 15:00 local time. Officials said there was no ongoing threat.
A second attack in less than three months
Analysis by Samantha Granville, US Capitol
Capitol Hill has been tense over the past three months. Since the riot on 6 January, the complex has been like a fortress with barbed wire, metal fencing and heightened security.
But just weeks later, we are back here with blocked roads, extra troops, and a solemn feeling.
Congress is in recess today and staff I have spoken to are grateful to be home and nervous about returning to work after the Easter holiday.
They say, understandably, that it is scary to have your workplace attacked twice in a short space of time.
It is concerning for them that even with ramped up security, an event that led to an officer's death still happened.
What do we know about the attack?
Shortly after 13:00 local time (18:00 GMT) the Capitol Police alert system sent an email to lawmakers and their staff ordering them to stay away from exterior windows and doors due to a threat. Anyone outside was instructed to seek cover.
At that time, a man driving a blue sedan had rammed the car into two officers standing at the North Barricade, according to police.
He exited the vehicle and ran towards the officers - at least one of whom drew a weapon and shot the suspect. The officers were then transported to hospital, one in an ambulance and one in a police cruiser.
Footage of the scene showed a helicopter flying overhead and what appeared to be two people on stretchers being moved into ambulances.
Onlookers were told to clear the area.
Police said the suspect died due to his injuries at 13:30 local time. Chief Contee told reporters the suspect does not appear to be known to Capitol or Metropolitan police, was not on any watch lists, and appeared to act alone.
Getty Images
The FBI's Washington Field Office is responding to the situation and is providing support to the Capitol Police. The US Attorney General is also aware of the incident and is being updated, according to CBS News.
The incident comes nearly three months after the deadly 6 January riot at the Capitol.
The force used on George Floyd on the day he died was "totally unnecessary", a senior homicide officer told the murder trial of Derek Chauvin.
Chauvin, 45, is accused of murdering Mr Floyd in Minneapolis last year by kneeling on the 46-year-old's neck for more than nine minutes, as he lay face-down in handcuffs.
Mr Floyd was being detained after he was accused of using a counterfeit $20 bill at a local shop.
Image:Richard Zimmerman was critical of the force used during George Floyd's death
In a half-day session on Friday, the jury heard from Lieutenant Richard Zimmerman, the head of the Minneapolis homicide department, who said: "If your knee is on a person's neck, that can kill him."
Lieutenant Zimmerman, who told the court he was trained in the use of force every year, also revealed that kneeling on the neck of a suspect was not something police officers are trained to do.
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He also told the jury that when a suspect is handcuffed and laying on their front, "that your muscles are pulling back... and if you're laying on your chest, that's constricting your breathing even more."
Prosecutor Matthew Frank asked the witness: "So in your opinion, should that restraint have stopped once he was handcuffed and thrown on the ground?"
More from Derek Chauvin
"Absolutely," Lieutenant Zimmerman replied.
The homicide chief also told the court that officers have a duty to provide care if the suspect is in distress, even if medical help is on the way.
Under cross-examination, the lieutenant was asked a raft of questions about the decision making officers use to decide how much force to use, including location, surroundings and the state of the suspect.
Defence lawyer Eric Nelson also asked Lieutenant Zimmerman if a handcuffed suspect could still pose a threat - to which he agreed that they could.
Image:Sergeant Jon Edwards was despatched to the scene as he began his night shift on 25 May
The court earlier heard from Sergeant Jon Edwards, who attended the scene of the incident as he began his night shift with the Minneapolis Police Department.
Sergeant Edwards explained that when he arrived at 38th and Chicago, Officers Lane and Kueng, who are also due to stand trial over the death of George Floyd, were still at the scene.
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George Floyd's brother on 'agony' of trial
He added that he told the pair to get out of their squad car and leave their belongings, after they were identified as being "involved officers".
Kueng and Lane were later taken to City Hall, as per procedure, and the squad car was seized.
Former police officer Chauvin has pleaded not guilty to murder and manslaughter charges, with his defence team arguing he was just doing what he was trained to do.
George Floyd Killing: The Trial - We will bring you live continuous coverage of court proceedings in the trial of Derek Chauvin on our website, app, YouTube and Sky Pop Up Channel on 524
A suspect has been shot dead and a police officer has been killed after a car rammed into a barricade near the US Capitol building.
Yogananda Pittman, acting chief of the Capitol Police, said the suspect jumped out of the vehicle brandishing a knife after he drove at the officers at around 1pm local time (6pm UK time).
She continued: "The suspect did start lunging towards US Capitol police officers, at which time US Capitol police officers fired upon the suspect.
"At this time the suspect has been pronounced deceased.
"Two US Capitol police officers were transported to two different hospitals and it is with a very heavy heart that I announce one of our officers has succumbed to his injuries."
Officials have said the suspect, who has not been identified, died in hospital.
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Ms Pittman said the suspect did not appear to have been on police radar.
Image:Members of the U.S. Capitol Police stand guard near the scene
Authorities added there was no longer an ongoing threat and that the attack did not appear to be related to terrorism.
There is also no immediate connection apparent between Friday's crash and Capitol riot on 6 January.
The crash and shooting happened at a security checkpoint near the Capitol as Congress is on recess.
House Speaker Nancy Pelosi has ordered the flags at the US Capitol to be flown at half-staff in memory of the officer who died.
Sky correspondent Alex Rossi, who is in Washington DC, said: "Police are carrying out forensic investigations, so they're gathering evidence, trying to understand exactly what has happened and why this has happened as well as how it was able to happen.
"Also, the FBI has now responded to this incident... they will be carrying out their own inquiries, trying to understand how and why this happened."
Image:A barrier was struck by a car near the Capitol building
Staff inside the building in Washington DC were sent a message telling them no entry or exit was allowed and they were warned to stay away from exterior windows and doors.
The message added: "If you are outside, seek cover."
The attacks comes as the Washington region remains on edge nearly three months after a mob of armed insurrectionists stormed the Capitol as Congress was voting to certify Joe Biden's presidential win.
Five people died in the riot, including Capitol Police officer Brian Sicknick, who was among a badly outnumbered force trying to fight off insurrectionists who supported former President Donald Trump's bid to overturn the election.
