Seventeen Marines suffered injuries in the rough landing of a helicopter troopship at a desert training area Friday afternoon, a Marine Corps spokesman said. Medical personnel late Friday were examining the extent of injuries to the Marines hurt in the 2:15 p.m. accident at the Marine Air-Ground Combat Center, Capt. Timothy Hoyle said. The injured, most of them based at Camp Pendleton, flew in another aircraft for treatment at the Navy hospital at Twentynine Palms, about 140 miles northeast of Los Angeles, Hoyle said. The helicopter, a CH-46E Sea Knight, and its crew of three belong to the 3rd Marine Aircraft Wing, headquartered at the Marine Corps Air Station in El Toro, Calif. Investigators searched for the cause of the accident, Hoyle said. The severity of damage to the aircraft was not immediately known. The 25-passenger Sea Knight, an assault helicopter in service since the Vietnam War, was participating in helicopter-borne assault training.