The steam pipe rupture that caused Thursday’s evacuation of the Longworth House Office Building was due to a deteriorated valve, according the Architect of the Capitol’s office.

Workers had turned off the steam to clean a radiator on the fifth floor when the valve failed and emitted a large amount of steam, AOC spokeswoman Eva Malecki said.

The steam set off an alarm shortly before 1:30 p.m. Thursday. The Capitol Police officers who initially responded thought it was a fire and evacuated the building.

“Unfortunately, when you have a building the age of Longworth — even though we do update those system components whenever we can — sometimes the component can deteriorate from the time you replaced or inspected it to the time when you go back and do the next round of inspection,” she said.

The deteriorated valve is partly a symptom of a $1.4 billion backlog in maintenance on Congress’ aging buildings. And the utility tunnels, which house steam and chilled water pipes, are one of the Capitol’s biggest problems.

For years, the AOC failed to fix several health and safety problems in the tunnels, resulting in a complaint from the Office of Compliance. The AOC is in the middle of a five-year abatement process that is expected to cost hundreds of millions of dollars.

Malecki said that process is focused on the main lines in the tunnels — not the pipes and radiators located in Congress’ buildings. A deteriorated valve is an issue of wear and tear, she said.

“It’s equipment and equipment fails occasionally,” she said.