A special effects assistant was critically injured Thursday after a simulated stinger missile exploded on the set of an upcoming Tom Hanks movie, authorities said.

Stars of Charlie Wilson's War including Hanks and Julia Roberts, were not on set at the time, Fire Capt. Darren Moon.

A group of special effects specialists were testing the handheld device when it exploded at Downey Studios, a former aerospace factory 10 miles southeast of downtown Los Angeles, Moon said.

The assistant was in critical condition while undergoing surgery Thursday evening at St. Francis Medical Center, a supervising nurse said. The assistant's name was not immediately released.

The cause of the explosion was under investigation. The device had been scheduled to be launched from a helicopter during filming Friday, Moon said.

The film is based on a nonfiction book about former congressman Charlie Wilson's covert dealings to fund the CIA's secret war against the former Soviet Union in Afghanistan during the 1980s.

Charlie Wilson's War was released December 21st, 2007 and stars Tom Hanks, Amy Adams, Julia Roberts, Philip Seymour Hoffman, Terry Bozeman, Brian Markinson, Jud Tylor, Hilary Salvatore. The film is directed by Mike Nichols.