"During ongoing asbestos abatement work there was a potential release affecting the House side of the Capitol," the office of the Architect of the Capitol said in a statement. "Samples have been collected to determine whether there was potential exposure."
Start of business was delayed until noon, from 10 a.m., Speaker John Boehner's office said.
Portions of the building remained closed even after normal operations resumed around 10 a.m., and would stay closed throughout the day as a hazardous material response team and Capitol Police investigated an incident that began sometime around 2:30 or 3 a.m.
The East Grand staircase will remain closed "until further notice."