Staff and visitors were evacuated from the Cannon House Office Thursday afternoon for about 45 minutes after alarms began blaring throughout the building.
The evacuation notice went out to staff via email just before 1 p.m.
The alarm stopped sounding by 1:14 p.m., but an all-clear was not issued. At 1:36 p.m., Cannon was cleared for re-entry.
The all-clear announcement was made via a loudspeaker and an announcement system, that's due for replacement.
Watch: Cannon House Office Building Evacuated in 2018
The exact cause of the alarm and evacuation has not been announced.
But personnel from Clark Construction, one of the contractors on the massive Cannon renovation project, were overheard taking responsibility for the triggered alarm outside of their field office in a Capitol Hill parking lot.
Architect of the Capitol staff said that the contractor will have to explain to AOC how they plan to prevent future similar incidents.
Cannon is undergoing massive renovations, and scores of construction workers were evacuated along with House staff and visitors.
Cannon is the oldest congressional office building, built in 1908. The building is currently undergoing a full renovation, expected to take 10 years and cost approximately $752.7 million.
Temperatures approached 70 degrees and many evacuees took advantage of the weather, bringing their lunches with them to eat outside. Any specific emergency gathering spots were not strictly enforced, with staffers sitting on the steps just outside entrances to eat and chat.
Capitol Police, an Architect of the Capitol Fire Marshal and other emergency management personnel were seen gathering at New Jersey Avenue and C Street Southeast with radios to discuss the issue.
Emails alerting staff of the evacuation told them "If nearby, take annunciators on the way out."
The pager-like devices are part of the Joint Audible Warning System that was introduced as a temporary measure following 9/11. House Sergeant-at-Arms Paul D. Irving told lawmakers this week that the annunciators are so old that batteries and other components are no longer produced. He has requested funds for a replacement system, which would include more than 2,500 devices in virtually every room on the House side of the Capitol.
With the new JAWS system, emergency voice notifications will be sent via secure radio frequency to all offices and meeting spaces throughout the campus, according to Irving. The JAWS works in conjunction with House Alert, which pushes alerts on desktop computers and cells phones and the loud speaker system in office building hallways.