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 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. 

Two law enforcement sources involved in the investigation told BBC partner CBS News that the suspect in the attack was 25-year-old Noah Green from Indiana.

They said that no prior information about him had been found on any police databases.

In a post from mid-March on a now-deleted Facebook page, Green wrote that he had recently left his job, "partly due to afflictions, but ultimately, in search of a spiritual journey". He added he had been suffering the "side effects of drugs I was intaking unknowingly" and wrote extensively of his interest in the black nationalist religious organisation, Nation of Islam.