From OPALCO:

At 5:44 p.m. on April 23, the power went out in San Juan, Island, Skagit and Whatcom counties due to a car accident on Highway 20 near Sedro-Wooley. Bonneville Power Administration (BPA)’s transmission line was knocked out – and at the same time Puget Sound Energy had taken a second transmission line down for planned maintenance. This brought the power capacity down dramatically and caused a cascading power failure throughout the North Puget Sound region. BPA brought power back up at about 9 p.m. and OPALCO had power restored to all members by 9:30 p.m.