Police were searching the Rayburn building in the Capitol complex Friday after reports of gunfire in a garage in the building.
"At 10:30 this morning, we received a report of shots fired inside the Rayburn garage," Sgt. Kimberly Schneider, a public information officer for the Capitol Police, said.
"At this time, we continue to investigate. We have tactical teams inside the building."
Armed officers were going door-to-door checking offices in the building.
CNN's Brian Todd reported at 1:30 p.m. that law enforcement officials said the search of the Rayburn building was "winding down."
Earlier, officials said two women reported a man with a gun inside a Rayburn gym.
A senior law enforcement source told Todd that police were investigating the possibility that the man may have been a plainclothes officer.
A woman was taken by ambulance from the House staff gym to the hospital during the lockdown, said David All, a spokesman for Rep. Jack Kingston, R-Georgia.
"She was not injured or shot, just a little shaken up under the circumstances," All said. "We have been in contact with our colleague and she is doing well."
CNN's Dana Bash said the Capitol was reopened by 12:45 p.m.
Law enforcement expert Mike Brooks said he had not heard any reports of victims or physical evidence of gunfire.
He noted that Capitol Police have a firing range in the basement, but a congressional source said the shots were not heard from that level.
Business continued as usual on the Senate side, Bash said.
CNN's Ted Barrett said people were moving outside the Rayburn about an hour after the report came in. That was "probably a good sign," he said.
Five ambulances, "a ton of police cars and a ton of cops" were also there, but there was no indication of any immediate threat, Barrett said.
"It certainly is calm and controlled at this point," he said. "The police do not seem overly concerned that there might be a gunman loose at this point."
The alert came moments after a press conference held by Rep. John Sensenbrenner on immigration reform, CNN producer Deidre Walsh said.
"We saw several Capitol Hill police officers running quickly down the hallway," she said. Soon after that an announcement was made throughout the building.
'No one's panicking'
Rep. Peter Hoekstra, R-Michigan, was conducting a House Intelligence Committee hearing at the time.
Hoekstra interrupted a witness to tell those at the hearing to remain in the room and said doors must be closed.
"It's a little unsettling to get a Blackberry message put in front of you that says there's gunfire in the building," he said.
"They said they heard gunfire in the Rayburn garage, but this is a huge building, I'm guessing it's a car backfiring or balloons popping," Gene Smith, chief of staff to Rep. Howard Berman, D-California, told The Associated Press. Berman has an office in Rayburn.
Rayburn halls were virtually empty and police were not allowing anyone to leave or take elevators or stairs to the garage, the AP reported.
"No one's panicking, everyone's calm," Charles Isom, spokesman for Rep. Chris Cannon, R-Utah, told AP. "It did ruin some people's lunch plans."