This April 7, 2015 cell phone image shows a State Department spokesperson giving the daily press briefing in relative darkness during a power outage in Washington, DC. [Photo/CFP]

WASHINGTON - A power line broke loose at an electrical substation near Washington on Tuesday, briefly dimming the White House, emptying museums and cutting electricity to government buildings and the US Capitol for hours.

The Justice Department and State Department were among thousands of customers whose power was cut in the early afternoon, along with the University of Maryland and some World Bank offices.

The outage was caused by a power line breaking loose in Maryland state south of Washington, causing a drop in voltage, electrical company officials said.

There were concerns that militant groups might be responsible, but the Department of Homeland Security said there was no indication of malicious activity. The outage was centered in the heart of Washington before power was largely restored a few hours later.

White House offices and the press room went dark briefly before backup sources kicked in. President Barack Obama was in the Oval Office and did not notice the power failure, spokesman Josh Earnest told a news briefing.

"Things are slowly but surely returning to normal here in the White House complex," he said.

The State Department's daily briefing was suspended after power was lost. At the US Capitol complex, where lawmakers were absent for an Easter recess, a backup generator switched on, keeping most lights and appliances running.