Two people, including a police officer, were killed after a vehicle rammed into a security access point outside the US Capitol in Washington on Friday.
Police say the person who crashed the vehicle was wielding a knife and was killed after being shot by authorities at the scene.
"At this time, [the situation] does not appear to be an ongoing threat," said acting Washington Metropolitan Police Department Chief Robert Contee. "It does not appear to be terrorism related."
Two police officers were injured in the attack and one succumbed to his injuries.
The dead police officer was confirmed "with profound sadness" by Capitol Police to be William "Billy" Evans, an officer who had served on the force for 18 years.
According to authorities, Evans was a member of the Capitol Division's First Responders Unit.
The second officer is in stable condition.
Law enforcement officials identified the deceased suspect as 25-year-old Noah Green. Agents are currently investigating his background to discern his motive for the attack.
Authorities were recently reported to be working to obtain warrants to access his online accounts.
Acting Capitol Police Chief Yogananda Pittman said the suspect did not appear to have been on the police radar.
"Jill and I were heartbroken to learn of the violent attack at a security checkpoint on the US Capitol grounds, which killed Officer William Evans of the US Capitol Police and left a fellow officer fighting for his life," US President Joe Biden said, referring to his wife, first lady Jill Biden.
He thanked Capitol Police and the National Guard Immediate Response Force who responded to the incident.
"We know what a difficult time this has been for the Capitol, everyone who works there and those who protect it," he said.
The US president said he has been receiving updates and will continue to receive details as the investigation develops.
Mr Biden ordered that White House flags be lowered to half-mast in honour of Evans.
Shortly after 1pm local time, police responded to "the North Barricade vehicle access point along Independence Avenue for reports someone rammed a vehicle into two officers".
The statement came after members of Congress and staff were told to shelter in place due to an "external security threat" via text alerts and sirens in Capitol buildings.
People were advised to stay away from exterior windows and doors, or "seek cover" if outside.
The Capitol complex remained on lockdown for two hours following the incident. All entrances and exits were sealed until shortly after police announced there was no ongoing threat.
Congressional offices were sent a message saying the "external threat" that led to the lockdown "has been neutralised".
Roads around the Capitol, Constitution Avenue between Second Street NE and First Street NW as well as First Street between Constitution Avenue NE and Independence Avenue SE, were closed following the incident.
National Guard and US Air Force personnel were deployed to the Capitol complex to support police.
Both the House and Senate are in recess at the moment.
The FBI said it is assisting police in the investigation.
This incident comes nearly three months after a violent mob of supporters of former president Donald Trump attacked the US Capitol, with the aim of overturning the 2020 presidential election.
The riot resulted in the deaths of five people, including a police officer.
The January 6 attack led to the deployment of thousands of National Guard troops in Washington and the placement of security fencing and barriers.
The security presence had decreased in recent months but had not fully returned to normal.
House Speaker Nancy Pelosi said Evans "is a martyr for our democracy" in what she called a "heinous attack".
She commended police for their work.
"Today, once again, these heroes risked their lives to protect our Capitol and our country with the same extraordinary selflessness and spirit of service seen on January 6," she said.
Ms Pelosi declared that Congress will work with law enforcement on the investigation.