Photos: Explosion rocks S.F.

Officers put up police tape around the site of a Friday morning explosion in downtown San Francisco. It may have been caused by a transformer below the street.

Police cordon off a street in downtown San Francisco after an explosion hit near Post and Kearny streets around 10 a.m.

Crowds gather to catch a glimpse of what caused an explosion near Post and Kearny streets in downtown San Francisco on Friday morning.

A television reporter interviews a witness after a downtown San Francisco blast blew off a manhole cover, hurt a woman and prompted police to shut down streets.