BOSTON (AP) — National Grid is investigating the cause of an outage that knocked out power to more than 44,000 of its customers north of Boston.

A utility spokeswoman says the problem first reported at about 7 a.m. Friday originated in an Everett substation, but the exact cause is not known.

The outages were concentrated in Malden, Medford and Everett, with virtually the entire city of Medford out, causing delays on the MBTA's Orange Line.

The spokeswoman says the number of outages was down to about 3,000 by 8:30 a.m.

National Grid has almost 1.3 million customers in Massachusetts.