US Capitol Building
US Capitol Building Thinkstock

The US Capitol Building and Capitol Visitor Center were evacuated on Tuesday afternoon after an alarm went off for reasons that are as yet still unknown.

Over a loudspeaker the emergency message, "evacuate, evacuate, evacuate the US Capitol Building," was heard.

Capitol Hill Police spokesperson Lt. Kimberly Schneider said that police and firefighting forces are still searching for the cause of the alarm.

Schneider added that there were no visible signs of smoke or fire.

Roads were closed in Washington DC as a result of the alarm, including First Street between Independence Avenue, SE and Constitution Avenue, NE, as well as East Capitol Street between First Street and Second Street.

The Capitol Building was empty of House representatives and senators at the time, given that they are on a week-long Memorial Day Weekend recess.

The mysterious incident comes just days after a pressure cooker was detonated by a bomb squad just outside the Capitol Building on Sunday, likewise causing road closures.

Officers found the pressure cooker in a car, although the incident apparently was not terror related.

The pressure cooker raised great concerns particularly since the appliance has been used in numerous terrorist attacks in the past as an explosive filled with nails and other shrapnel to inflict gruesome casualties.

The most notable usage as a means of attack was seen in the 2013 Boston Marathon bombing, in which an attack with two pressure cookers murdered three and wounded over 260 others.