Police shot an armed man as he tried to enter the US Capitol building’s visitor centre in Washington, officials said.
A female bystander suffered minor injuries when police fired the suspect and she has been taken to hospital.
Officials said the suspect was known to Capitol police and ABC News reported that the man had disrupted a House session last year.
The massive complex was placed on lockdown, but the security precaution has since been lifted.
The Capitol Visitor Center is still closed, but the House and Senate have reopened for official business only.
Washington’s city police called it an “isolated” incident, saying there is “no active threat” to the public.
Officials originally said a police officer had been hurt, but Capitol Police Chief Matthew Verderosa said no officers were shot.
The gunman was undergoing surgery at a hospital and his condition was unknown, Mr Verderosa said.
US Congress is on recess for the Easter holiday, but some members of Congress and staff members were on site.
To enter the US Capitol or any Senate or House office buildings, visitors must go through a metal detector and weapons are not permitted.
Mr Verderosa said the suspect pulled out his gun just before being screened by those metal detectors.
Diane Bilo, a woman from Ohio, told the Washington Post her husband heard the shot.
“My husband said he heard a shot followed by a full clip,” she said.
Multiple members of Congress have posted to social media, reporting that their staff were safe.
In 1998, an attacker shot and killed two police officers at the Capitol. The visitor centre was built as a way to strengthen security after the shooting.
Many tourists are in Washington for the annual Cherry Blossom festival – about 1.5 million people visit the city during the four weeks of the festival.
The White House was also put on lockdown temporarily due to a separate incident.