The four crew members aboard a U.S. Marine Corps helicopter that crashed in Southern California on Tuesday are presumed dead, authorities said.

A CH-53E Super Stallion helicopter from the 3rd Marine Aircraft Wing went down “in the vicinity” of El Centro during a routine training mission, according to a statement from the Miramar air station near San Diego. The crash occurred at approximately 2:35 p.m., the statement said.

The cause of the crash is currently under investigation, officials said. The names of the crew members are being withheld pending notification of family members, according to the statement.

Marine officials have not identified the crew’s squadron or the base from which it flew, according Capt. Morgan Frazer, spokeswoman for the 3rd Marine Aircraft Wing.

The Naval Air Facility El Centro, “along with other agencies to the scene,” responded to the crash, according to a Facebook post. Several other helicopters in the area also assisted with the incident, the post said.

Tuesday’s crash would be the deadliest Marine aircraft crash since a KC-130T transport plane went down in Mississippi last summer, killing 15 Marines and a Navy sailor, according to the Associated Press. The Washington Post reported that the July incident was “the worst aviation accident for the branch in more than a decade.”

Other crashes involving CH-53E helicopters include an incident last October in Okinawa, but there were no injuries, the AP reported. In 2005, more than 30 people died when another CH-53E helicopter went down in Iraq during a sandstorm, according to AP.

There are currently more than 150 of these types of helicopters in operation, according to Military.com. The aircraft is the “Marine Corps’ primary heavy lift helicopter and has been in service for over 30 years,” the website states.