Police also closed Pennsylvania Avenue in front of the White House.
CNN's Athena Jones, who was at a Senate office building near the Capitol, said she heard gunshots that sounded like fireworks.
The FBI dispatched units in response to reports of shots near Garfield Circle, which is on the Senate side of the Capitol.
CNN Chief Congressional Correspondent Dana Bash learned that one person was injured. No further details were immediately available.
House and Senate sessions were immediately suspended.
A Capital Police bulletin said reports of gunshots required "all occupants in all House office buildings to shelter in place."
"Close, lock and stay away from external doors and windows," the bulletin said.
Authorities later lifted the lockdown, Bash later reported.