A power outage temporarily blackened Washington, D.C. this afternoon, interrupting a State Department briefing and forcing the White House onto back-up generators. Traffic lights also went out in parts of the city. The chief spokesman for 1600 Pennsylvania Avenue confirmed that the outage 'briefly had an impact on the White House complex' but it was 'back on the regular power source' by the time he began his briefing approximately an hour and a half later. 'Things are slowly but surely returning to normal at the White House complex,' Josh Earnest, White House press secretary, said, noting that he was in the Oval Office with the president when it happened, and it was barely noticeable. Scroll down for videos . District of Columbia Metro Police and U.S. Secret Service Uniform Division officers clear Pennsylvania Avenue in front of the White House. The temporary power outage affected the State Department, the Department of Energy, the University of Maryland in College Park and other areas in the District of Columbia and Maryland . With traffic lights still out at a busy intersection not far from the White House, a Metro Police Department officer directs vehicles after an electrical blackout affected the city . Several Smithsonian museums were affected as well, including the Portrait Gallery, and guests were evacuated . The impact on the White House was 'minimal,' he said, but was unable to say how many offices in the West Wing were affected. The source of outage was not immediately known, but the Department of Homeland Security has said there's no evidence of malicious intent or terrorism related activity. A possible cause was an explosion at a power at a plant located in southern Maryland, according to the Associated Press. Power company Pepco told local news station WTOP that a dip in power in the transmission line at one of its Maryland stations was responsible. As of a 1:35 pm update, more than Washington-area customers - including government offices - had reported outages, Pepco said. Visitors wait for the Smithsonian Air and Space Museum to re-open after it was evacuated following a power failure . A security guard announces the museum's closure to prospective visitors . The outage lasted but a few seconds at the White House, as the government building immediately moved onto generators. Press Secretary Josh Earnest's regular briefing with reporters was pushed off schedule by more than an hour. An unrelated matter, said by the White House to be an 'unattended package,' forced the building into a temporary lock down, adding to delays. Other government agencies, including the State Department, were affected for longer periods of time than 1600 Pennsylvania Avenue. The State Department's acting spokeswoman Marie Harf was in the middle of her daily briefing when the blackout hit. She lit the room with her cell phone, photos posted to Twitter show, and ultimately had to cut it short. Oprah Winfrey also got caught up in the power outage while speaking at the Maya Angelou Forever Stamp Dedication at Warner Theatre in downtown D.C. The U.S. Capitol, the Justice Department, IRS, Department of Energy and several Smithsonian museums, as well as the city's metro system were also affected, according to reports. Both the National Air and Space Museum and the National Portrait Gallery were evacuated as a result. Some stations of the city's metro system were running on emergency lighting, CNBC reported, and the Capitol complex's power system also failed over to a generator. Oprah Winfrey was in the middle of a speech at a dedication ceremony for the Maya Angelou Forever Stamp when the lights went out on her at Warner Theater in downtown D.C. The power failure extended as far northeast of Washington as the University of Maryland's flagship location in College Park, located roughly 10 miles outside the heart of D.C. Other government agencies, including the State Department, were affected for longer periods of time than 1600 Pennsylvania Avenue. The State Department's acting spokeswoman Marie Harf was in the middle of her daily briefing when the blackout hit. She lit the room with her cell phone, photos posted to Twitter show .
It interrupted a State Department briefing and forced the White House onto back-up generators .
Outage 'briefly had an impact on the White House complex' but it was 'back on the regular power source' an hour and a half later .
White House press secretary says he was in the Oval with the president when it happened, and it was barely noticeable .
Related to a dip in power in a transmission line at a Maryland facility, power company says; another report says it may have been an explosion .
Homeland Security says there's no evidence of malicious activity .