WASHINGTON (AP) – Heavily armed Capitol police swarmed through a House office building and briefly closed the Capitol on Friday after a congressman thought he heard gunfire and triggered what turned out to be a frightening false alarm.

Authorities reported no arrests, gunfire or even injuries after a methodical three-hour search that stranded lawmakers and aides alike in their offices – though one aide was so rattled by the sight of police tactical teams that she was taken to a nearby hospital.

Through the day, officers with rifles stood by outside, and ambulances arrived after the phoned-in report of gunfire in the garage of the Rayburn House Office Building.

The report originated with Rep. Jim Saxton, R-N.J., his press secretary said. Saxton heard what he thought were gunshots and had a member of his staff call Capitol Police, said spokesman Greg Keeley.

“I heard what I thought to be between six and ten shots. It sounded exactly like gunfire to me,” Saxton told Fox News Channel after the search was concluded and the huge office building was reopened.

Capitol Police Sgt. Kimberly Schneider said officers were looking at a “plausible explanation” that the day’s events resulted from noise made by construction workers in the Rayburn garage. “In doing their routine duties, they made some sort of a noise that sounded like shots fired. So it was a valid call,” she said.

On high alert, police lined the street between the Capitol and the Rayburn building, rifles prominently displayed, and four ambulances, two fire trucks and other emergency vehicles were on the scene. Police methodically searched the sprawling building, where congressional staff members had locked themselves into their offices as a precaution.

Before the all-clear, Schneider said, “Lives could be at risk. If we have a gunman in the building we certainly want to find him. It’s premature to assume that it may not be a gunman.”

The Senate was in session at the time, but the House was not as most lawmakers had left for the Memorial Day recess.

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

Indeed, one congressional staff member was taken to a hospital after suffering a panic attack during the lockdown and search, Schneider said. The woman was released a short time later.

The search was a complicated one. The building, which covers a long city block, is connected to a second office building by an underground tunnel. That building, in turn, is connected to the Capitol by a second underground tunnel.

The Rayburn House Office Building was completed in early 1965 and is the third of three constructed for the House of Representatives. It has four stories above ground, two basements and three levels of underground garage space.

Nearly two hours after the first alert, Capitol police sent an e-mail message to occupants of the office building saying they would soon begin a floor-by-floor search.

“During the search, the police officers will knock three times on each office door, announce ‘United States Capitol Police,’ knock three additional times,” and then confirm their identity by speaking a code word, it said.

Usually teeming with visitors, Capitol Hill was uncharacteristically quiet. Although it was the start of a holiday weekend, tourists were few. Independence Avenue, which runs between the Rayburn building and the Capitol, was closed to traffic. Yellow caution tape strung from light posts cordoned off an area filled with emergency vehicles.

The Capitol itself was all but deserted even though it remained open for official business. Anyone approaching it was asked for identification. No visitors were allowed in. Many lawmakers and staff members had left for a weeklong break.

The lockdown left dozens of people stuck in the Rayburn building for the better part of the day. As the lockdown continued past the lunch hour, staffers in several offices were raiding their refrigerators to settle their hunger. In one office, staffers hunkered down with the doors locked and deadbolted, staying away from the windows as they waited for the building to reopen.

Cathy Travis, a spokeswoman for Rep. Solomon Ortiz, D-Texas, was in his fourth-floor office at 1 p.m. when “four or five really huge, burly young cops did a boom, boom, boom on the front door and walked in.”

She said they told her and others to stay where they were and stay out of the way for a search that she said didn’t seem to take more than a minute as they checked IDs and all parts of the office.

An Associated Press reporter heard noise outside a Rayburn press room and peeked out the door. A police officer, gun drawn, shouted, “Get in the room. Get in the room.”

Associated Press Writers Jim Abrams, Katherine Shrader, Suzanne Gamboa, David Espo, John Heilprin, Mary Dalrymple, Andrew Miga, Lara Jakes Jordan and Sam Hananel contributed to this report.

AP-ES-05-26-06 1819EDT