A Marine helicopter has crashed during a southern California training mission and authorities believe all four crew members were killed, reports the AP. The military says the CH-53E Super Stallion went down shortly after 2:30pm Tuesday near El Centro. The helicopter was with the 3rd Marine Aircraft Wing out of the Miramar air station near San Diego. The cause of the crash is under investigation. Authorities say the names of the dead are being withheld pending notification of the families, reports the San Diego Union-Tribune. (More US military stories.)