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The Capitol went into lockdown over the incident. Journalists at the complex filmed both the scene of the crash and a US Park Police helicopter touching down on the lawn outside the east entrance.

"It is with a very very heavy heart, that I announce one of our officers has succumbed to his injuries," Acting Chief Yogananda Pittman told reporters at a press conference on Friday afternoon, asking the public to keep his family and Capitol Police in their prayers.

"The suspect exited the vehicle with a knife in hand," Pittman said. He "did not respond to verbal commands" and "started to lunge" towards the officers, at which time they shot him.

The incident "does not appear to be terrorism-related", but remains under investigation, said DC Metropolitan Police Chief Robert Contee. Capitol Police did not have a previous record of the suspect, suggesting there was no "nexus" with any member of Congress.

Law enforcement and first responders have swarmed the gates of the US Capitol after a car rammed a security barrier, injuring two officers. Footage shows the chaotic aftermath with police swooping in and a helicopter landing.

"Someone rammed a vehicle into two" officers at the checkpoint on Constitution Avenue shortly after 1pm on Friday, Capitol Police said, adding that "a suspect is in custody." The two officers are injured, and all three have been transported to the hospital, according to police.

Media reports suggest that the suspect may have been wielding a knife, and was shot and critically injured by the officers. Multiple reports say he has since died.

(Warning: graphic)

Video footage shows the injured suspect being led away on a stretcher, while alternate shots show someone who appears to be a police officer being wheeled away by medics.

The response by law enforcement was swift and heavy. A helicopter was seen swooping down onto Capitol grounds, and National Guard soldiers - deployed in Washington, DC, since the pro-Trump riot on Capitol Hill in January - could be seen running to the scene of the incident.

Streets around the Capitol have been sealed off by police, and the building itself placed on lockdown.

Security around the Capitol has remained elevated since the January riot, with Democrats insisting that the installation of a high-security fence around the building was necessary to thwart would-be "domestic terrorists." However, the motive of Friday's attacker remains unknown for the moment.

Congress is currently on recess, and few lawmakers were likely in the building at the time. President Joe Biden departed Washington earlier on Friday, en route to Camp David for the weekend.

Metropolitan Police Chief Robert Contee said there no longer appeared to be an ongoing threat.

"It does not appear to be terrorism-related, but obviously we'll continue to investigate to see if there is some type of nexus along those lines," Contee said.

© NBC News/OpenStreetMapLocation of the Washington DC car ramming incident

Several streets in the area were shut down:

• Constitution Avenue between Second Street NE and First Street NW
• First Street between Constitution Avenue NE and Independence Avenue SE
• East Capitol Street between First Street and Second Street

A lockdown of the U.S. Capitol complex has been lifted.

During the lockdown, those outside were told to "seek cover," according to an email from U.S. Capitol Police. Those inside were permitted to move around within the buildings and underground between buildings.

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