A hazardous materials response team was in the building following an incident that began at around 2:30 a.m. or 3 a.m., Capitol Police said. There were no initial reports of any injuries.
By mid-morning, most of the building was reopened and Capitol tours on the House side resumed. The Senate, at the other end of the 751-foot-long building, seemed unaffected by the incident.
The East Grand Staircase, which runs from the first floor to the third floor inside the House side of the building, was blocked off, and more than a dozen workers were gathered there. Also closed was the Thomas P. O'Neill Jr. Room, a third-floor room near that staircase that was named for the late speaker and Massachusetts Democrat.
The office of the Architect of the Capitol, which oversees the Capitol and other nearby buildings, released a statement indicating the incident involved possible asbestos exposure.
"During ongoing asbestos abatement work there was a potential release affecting the House side of the Capitol," the statement read. "Samples have been collected to determine whether there was potential exposure."
The House began the day's session at noon instead of 10 a.m. because of "an industrial accident," according to a statement from House Speaker John Boehner, R-Ohio. Even so, by midmorning a handful of tourists were sitting in the visitors' gallery, observing an otherwise empty chamber.
The Senate began its session as scheduled at 10 a.m.
Construction of the main, center section of the Capitol began in 1793 and was finished in 1826.
As the country grew and more lawmakers joined Congress, a south wing for the current House chamber and a north wing for the Senate were built. Both were completed in 1868, along with a new, larger dome.
The architect's office has been repairing decaying plaster throughout the building. It has also started preparations for a project to repair the 8.9 million pound, cast iron dome.