The U.S. Capitol was evacuated shortly after noon Monday after the building lost power.

Electricity was restored about half an hour later but officials decided to keep the building evacuated until the cause of the outage was determined, said Bob Stevenson, a spokesman for Senate Majority Leader Bill Frist, R-Tenn.

Mary-Beth Hutchinson, a spokeswoman for Potomac Electric Power Co., said the electricity was shut off automatically after there was "a momentary drop in voltage due to customer operations up the lines" away from the Capitol.

"The protective equipment sensed the significant change in voltage and tripped," she said.

Nearby office buildings were not affected by the outage. Before the evacuation sirens went off, more than 100 visitors sat in the darkened House gallery. They exited with everyone else when the alarm sounded, calmly walking toward exits.