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 . Comment: