A Marine helicopter crashed during a Southern California training mission and all four crew members were believed killed, the military said.
The CH-53E Super Stallion went down shortly after 2:30 p.m. Tuesday near El Centro, a few miles from the U.S.-Mexico border. El Centro is approximately 113 miles east of San Diego, CBS San Diego affiliate KFMB-TV notes.
The helicopter was with the 3rd Marine Aircraft Wing out of the Miramar air station in San Diego, according to a base press statement.
The nearby Naval Air Facility El Centro messaged that the crash site was north of Plaster City, west of El Centro.
The cause of the crash is under investigation.
The names of the crewmembers will be withheld until 24 hours after their relatives are notified, the Miramar base said.
It would be the deadliest Marine aircraft crash since a KC130T transport plane went down in Mississippi last July, killing 15 Marines and a sailor.
The CH-53E Super Stallion is the largest helicopter in the U.S. military. It is used for minesweeping and transport and can carry dozens of troops and tons of cargo.
Last October, a CH-53E helicopter crashed and burned in Okinawa but nobody was injured.
The Associated Press contributed to this report.