Capitol Police identified the slain officer as William "Billy" Evans, an honored 18-year veteran of the force.
Evans started his service on March 7, 2003, acting Capitol Police Chief Yogananda Pittman noted in a statement, and he was a member of the Capitol Division's First Responder's Unit. No other details were provided at the time.
The other police officer is in "stable and non-threatening condition," Capitol Police revealed.
The suspect — who has since been pronounced dead — has been identified as 25-year-old Noah Green, according to two sources briefed on the details of the investigation.
He entered the north barricade of the Capitol just after 1 p.m. ET, ramming his car into two officers and then crashing into a barrier, Pittman stated at a briefing.
Officers engaged the suspect, she added, but he failed to respond to verbal commands. He exited the vehicle and began "lunging" toward police with a knife visible in his hand, at which point officers fired their weapons, killing the suspect.
"This has been an extremely difficult time for the U.S. Capitol Police after the events of Jan. 6, and now after the events that have occurred here today," she said. "So I ask that you keep our U.S. Capitol Police family in your thoughts and prayers."
Little is known about the suspect or possible motive, authorities said, as the investigation remains in its early stages.
Pittman revealed Capitol Police did not have the suspect on file, so there is at present no indication that "there's any nexus to any member of Congress."
Video from reporters at the scene posted on Twitter appeared to show a car that had crashed into a barrier. In the videos, at least two people were carried on stretchers into waiting ambulances.
The Capitol was placed under lockdown amid what Capitol Police described as "an external security threat."
No entry or exit was permitted, and police declared: "If you are outside, seek cover."
Capitol Police tweeted just after 3 p.m. that it had cleared the "external security incident" at all U.S. Capitol campus buildings but said the area around the scene would remain restricted. The attack occurred as the House and Senate were in recess, though both buildings have staff – and reporters — working inside. Several lawmakers took to Twitter to share thoughts and prayers for those at the Capitol as the situation unfolded.
A statement from the Washington, D.C., Metropolitan Police Department said: "A call came in at approximately 1:05 pm to the unit block of Constitution Ave, NE, for the report of a possible shooting." Washington police provided no other details.
The FBI's Washington Field Office announced it had "responded to an incident in the U.S. Capitol" and is providing support to Capitol Police.