Two are dead after a man crashed a vehicle into a police car near the US Capitol - the driver and a police officer.
The Capitol went into lockdown on Saturday morning (NZ time) when a car smashed into a police vehicle forming part of a barricade about 100m from the entrance, CNN reported.
"Both officers are injured. All three have been transported to the hospital," an earlier tweet from police said.
Several streets were closed, authorities fearing a repeat of the carnage that swamped the Capitol on January 6, when disgruntled supporters of outgoing President Donald Trump stormed the heart of US democracy, leaving five people dead.
The driver reportedly got out of the vehicle and stabbed one of the police officers. The Associated Press and others later reported the driver had died, then an officer's death was announced at a press conference just before 8am (NZ time).
The National Guard was deployed, CNN said, and anyone inside the Capitol building itself told to say where they were.
The lockdown was lifted about 8:20am (NZ time), police having determined the threat "has concluded".
Joe Biden wasn't at the Capitol at the time - he's at Camp David, a country retreat used by US Presidents. House leader Nancy Pelosi ordered flags at the Capitol to be flown half-mast in honour of the killed officer.
"Praying for the United States Capitol Police officers who were attacked at the Capitol," Republican leader Mitch McConnell wrote on Twitter.
"There is a sense that now ... simply going to work is something that has become dangerous," Democratic representative Ro Khanna told CNN, saying it reminded him of January 6.
"I can't imagine saying that, that going to the United States Capitol to represent your constituents, is actually a dangerous thing in the United States of America. It's just deeply saddening."
Police said the incident did not appear to be terrorism-related.