The Senate canceled its Monday session and it and its adjacent office buildings were put into lockdown after fatal shootings at the nearby Navy Yard in Washington.
Workers in the Senate buildings were not allowed to leave and nobody was allowed to enter, the Senate Sergeant at Arms said in a statement.
Shortly afterward, the Sergeant at Arms announced that employees were being allowed to leave their offices, but nobody other than staff would be allowed to enter the Senate buildings until Tuesday morning.
"We do not have any information to suggest the Senate, its members, or staff are in any danger, but out of an abundance of caution, we feel this is the best course of action to keep everyone safe," said Sergeant at Arms Terrance Gainer.
The Senate canceled its Monday session because of the Navy Yard shooting.
The House of Representatives, which was not in a work session on Monday, did not receive any lock-down notice, according to officials.
Gainer said the decision to lock down Senate buildings was made because of the "possibility of suspects remaining at large."
At least 13 people were killed in the shooting at the Navy Yard, which is less than two miles from the Capitol building. One suspected gunman was among the dead and authorities were searching for two other possible assailants wearing military-style clothing, officials said.