Reports of shots fired inside a House office building on Capitol Hill — which caused a shutdown Friday of that building as well as the Capitol — likely resulted from noise caused by construction workers inside a parking garage, a police spokeswoman said.

Calling it a "plausible explanation," U.S. Capitol Police spokeswoman Kimberly Schneider said workers were doing their routine duties when they made some kind of noise.

"It was a valid call. ... It sounded like gunfire and we responded as such," Schneider said. Without expanding further, she said it was a congressional employee who reported the incident. It wasn't clear what type of work the construction crew was doing.

"This is very good news, and we're happy to report it to you," Schneider said, adding that the investigators are continuing to gather information.

Police issued an all-clear alert Friday afternoon after investigating reports of gunshots heard earlier in the day in the garage level of a House building.

Police opened the Rayburn House Office Building after sealing it for more than four hours to search the garage area for a gunman or anyone who may have been injured after smelling gunpowder and receiving reports of shots.

Elevator contractor Greg Wood told FOX News that he had been working in the Rayburn building garage at about the time the shots first were reported. Once police said the building could reopen, they asked him to drive his forklift over a speed bump and rumble strips inside the garage.

Wood said he wouldn't be surprised if someone misconstrued the loud bangs of the forklift arms bouncing on the ground as gunfire.

"Right as you drive in the garage there's a speed bump and some rumble strips and anyone who's ever driven a fork lift knows there are hard rubber tires and no suspension, so when I hit the rumble strips the thing goes, 'Bang! Bang! Bang! Bang! Bang!' " Wood said.

Earlier on Friday, law enforcement officials were "responding to reports of shots fired inside the Rayburn garage," Schneider said. "We're handling this as a very serious matter."

There were independent reports by staff and a police officer that they had heard gunshots, FOX News confirmed. The Capitol was reopened for a second time at about 12:30 p.m. EDT after being locked down, according to police.

House staffers received an e-mail message at 10:38 a.m. saying police were investigating reports of gunfire. The internal Capitol voice alarm system said police were looking into "the sound of gunfire in the garage level of the Rayburn House Office Building."

The message informed staffers in Rayburn to stay where they were and “quickly move into the nearest interior office space or interior hallway and away from windows.”

Staffers were also told to find their emergency kits and personal belongings, close the doors behind them and remain calm. Officers were searching the building room by room.

Ambulances arrived on the scene and emergency workers took a woman suffering from anxiety-related health problems out on a gurney from the building, according to a FOX News producer on the scene. There were no reports of other injuries or arrests.

Members of the FBI terrorism task force were sent to the scene.

"The FBI always offers assistance and our response in this situation is standard protocol," FBI Special Agent Richard Kolko said.

Police were alerted after someone called the Capitol Police dispatch center to report gunfire, Schneider said.

The Senate was in session at the time of the incident, but the House was not.

Rep. Peter Hoekstra, R-Mich., conducting a House Intelligence Committee hearing, interrupted a witness to request those attending the meeting to remain in the room and said the 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.

Jeff Connor, a spokesman for Rep. Jo Ann Emerson, R-Mo., said Capitol Hill police notified the office that gunfire was heard in the garage.

"They specifically said there was the sound of gunfire on one of the garage levels of the Rayburn House office building and asked staff to remain in their offices," Connor said.

Staffers and other Capitol visitors who were in the basement at the time were locked down in the area while others were evacuated from the building, according to a Capitol police officer.

"They want us to stay in our offices and I'm perfectly happy to do that, but I don't feel endangered," said Gene Smith, chief of staff to Rep. Howard Berman, D-Calif., who has an office in Rayburn.

"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."

There is a firing range in the basement of the Rayburn building for Capitol Police to use.

Many members of the House were back home in their districts for the Memorial Day recess. House Speaker Dennis Hastert was home in Illinois when the incident began, an aide in his office said.

Bradshaw Smith, a tourist from Branson, Mo., who was in the Rayburn building cafeteria on a visit to the Capitol, said police yelled at bystanders to go to their offices.

"I weigh about 300 pounds but I learned to run pretty fast," Smith said. "This has been a little more excitement than what we were anticipating."

In 1998, a gunman burst through a security checkpoint at a Capitol entrance and killed two police officers and injured one tourist in a shootout. Russell E. Weston Jr., the 41-year-old gunman who had a history of mental problems, was wounded in the assault. Weston was declared incompetent to stand trial and is in a federal prison hospital.

FOX News' Jim Angle, Melissa Drosjack, Major Garrett, Molly Henneberg, Catherine Herridge, Molly Hooper, Jim Mills and Greg Simmons and The Associated Press contributed to this report.