A special effects technician rigging an explosive for a movie car chase scene was seriously injured Friday when the charge detonated, authorities said.

The blast injured Grant Richens, 37, of Pleasant Grove, who was flown to LDS Hospital in Salt Lake City with a broken leg and lacerations on both arms and legs, said hospital spokesman Tim Madden.The production company, Action Films, was shooting footage for "some sort of karate film," in a gravel pit here when the accident occurred shortly after 1 p.m., said Summit County Sheriff's Sgt. Larry Henley.

He said the victim apparently was stringing explosives that accidentally detonated.

Henley said the cause of the blast was not immediately known.