Most of the 39,000 customers who lost electricity in a power failure in Washington on Saturday had their service restored after about four hours, officials said.

As of 6:30 p.m., 5,500 customers were still without power, according to Potomac Electric Power Company.

Frank Tedesco, a spokesman for the company, attributed the failure to an issue with electrical equipment at its Florida Avenue substation. Officials did not know the cause of the blackout or whether it was related to the heat. Temperatures were in the low 90s around 2:30 p.m. when the power went out.

The company said most of the customers affected were in the Mount Pleasant and Columbia Heights neighborhoods. The White House was unaffected by the power failure.