The suspect was shot by police after he got out of a car and attacked police. He later died at a nearby hospital.
Police officer William "Billy" Evans, an 18-year-old veteran of the police force, was identified as a police officer killed on Friday. Capitol police said one police officer was in a stable condition with non-life-threatening injuries.
Legal sources told CBS News that the suspect was identified as Noah R. Green, a 25-year-old Indiana man, who was not on the legal radar.
Green, an African American man, was not on any watch list and appeared to be working alone, according to a state law official and a source for Capitol Hill.
Police said Green crashed his car into police at 1:02 p.m. then he got out of the car and threw himself at them. The officers then fired shots at the suspect, Capitol Chief Executive Officer Yogananda Pittman said at a media briefing on Friday. Several sources said one of the officers was stabbed in the face, but it was not immediately clear which officer was stabbed.
Just after 1 p.m., Capitol Police sent an initial warning to conference staff, warning them of "an external security threat." Within an hour and a half, police sent out another warning saying the threat was "out of control".
In a statement, President Biden said he and his first wife Jill Biden were "saddened" by the news of the attack, adding that he was receiving information about a security adviser in his country. Mr Biden, who was at Camp David over the weekend, ordered the White House flags to be reduced to half staff.
Speaker Nancy Pelosi said in a letter to her colleagues that she was ordering the Capitol flags to be lowered to half the staff.
"In the face of such suffering, there are not enough words. However, I hope it is a comfort to the family of Officer Evans and the family of another officer who was injured on the job that many are saddened and thank the officials for 'Capitol's brave protection," Pelosi said. "May it be a consolation to Officer Evans and his family that the American flag is flown to half a half to pay tribute to the self-sacrificing spirit of all Capitol police, and that the president has appointed this White House and all state buildings."
Capitol police said the Evans family was requesting privacy at the time.
The incident took place about three months after protesters overtook the U.S. Capitol during an attack on January 6. The riots resulted in the deaths of five people, including Capitol Police Officer Brian Sicknick. Two Capitol police officers also committed suicide after the attack, injuring scores of police officers.
The union representing the police called Friday's attack "a tragedy that adds a very difficult year to our membership."
In a statement sent to CBS News, Sicknick's girlfriend, Sandra Garza, said it was "heartbreaking" for Evans' family and Capitol police.
"Sadly, I understand the conflict, shock and great pain Evans' family feels and is experiencing," Garza said. "I do not believe this is happening again, and soon after the death of Brian and Officer Liebengood. Capitol police have gone through a lot and have begun the process of recovery since the Capitol attack on January 6. I hope the other injured officer will fully recover and reunite with his family as soon as possible."
Following the January attack, a fence around the Capitol buildings was erected and thousands of National Soldiers were sent to Washington. That security fence was lowered last week, although the inner fence layer surrounding the Capitol building itself is still intact.
The bar where the incident took place is a checkpoint on the Senate side of the Capitol. It was set up after a terrorist attack on 9/11 to protect the Capitol from car bombings, said Congressman Tim Ryan, who heads the Capitol Police Committee.
Senator Roy Blunt, Senator Chris Van Hollen and Eleanor Holmes Norton Opposition to the Department of Development Development introduced a bipartisan law, a bicameral law in March to prevent government subsidies from being used to permanently enclose the Capitol. However, Friday's incident could delay efforts to remove an internal barrier around the Capitol. Ryan said legislators would be "reviewing everything" after the attack.
"I think we're going to review everything this time around, including the fence, and the biggest concern I know about most people is this kind of lone wolf attack," Ryan told a news conference Friday. "The fence is a nuisance, it's boring, it's boring. No one wants it there. But the question is, is the environment safe enough to be able to remove it? In the meantime, maybe that fence can prevent some of these things from happening."
There were fewer people than usual at the Capitol on Friday as Congress is on vacation; most lawmakers are in their homelands. Some conference staff, as well as journalists and police, were present in their offices.
Andres Triay, Michael Kaplan, Kris Van Cleave and Rebecca Kaplan contributed to the report.