Watch: Massive explosion at Philly plant

A major fire that broke out at a refinery complex in Philadelphia burned for more than six hours. Four employees sustained minor injuries in the mishap. The exact cause of fire is unknown.

Philadelphia:A fire at a refinery complex in Philadelphia burned for more than six hours after it began.

Philadelphia Energy Solutions says it has not determined what caught fire, but it believes it was mostly propane.

Earlier Friday, a Philadelphia Fire Department official had said a vat of butane caught fire.

The company says there were three separate explosions, which were felt miles away.

Four employees sustained minor injuries and were treated at the scene.

An order for nearby residents to shelter in place was lifted after 7 a.m.

It's the second fire at the refinery in the past month, following a June 10 fire in which no injuries were reported.