Five US Marines are missing after their helicopter went down in the snow-covered California mountains.
The CH-53E Super Stallion was found about 45 miles (72km) from its destination after the team were "reported overdue".
But the fate of the five Marines travelling on board is unclear.
They were on a training flight to Marine Corps Air Station Miramar, near San Diego, early on Wednesday.
The Marine Corps said the troops were on a flight from Las Vegas to Miramar.
They are assigned to Marine Heavy Helicopter Squadron 361, part of the 3rd Marine Aircraft Wing.
Fire crews were alerted to the missing aircraft at 02:20 local time on Wednesday (10:20 GMT). Crews were dispatched to an area near Lake Morena, California, before locating the aircraft hours later in Pine Valley in the Cuyamaca Mountains.
Rescuers were "using ground and aviation assets to locate the aircrew in co-ordination with the San Diego County Sheriff's Department and multiple federal, state and local agencies", the 3rd Marine Aircraft Wing said in a statement.
The search efforts have been hindered by heavy snow and wintry conditions in the area, officials said. The incident comes in the wake of a winter storm that has dumped heavy rain and several inches of snow in mountainous areas.
First introduced to active service in 1981, the CH-53E Super Stallion is considered a mainstay of Marine Corps aviation and usually serves in a transport role. It can carry up to 37 passengers.
In addition to carrying troops, the helicopter - which is also used by the US and Japanese navies - can be used in combat and to transport supplies.
The Marine Corps Super Stallion has a chequered safety record. There have been several serious accidents involving the helicopter in recent years.
In April 2018, four troops were killed when their Super Stallion crashed near El Centro, California, while on a training mission.
In a separate incident in January 2016, 12 Marines were killed when two Super Stallions collided over the Pacific near Oahu in Hawaii.
This is not the first accident to occur in these California mountains recently.
In August, a Marine Corps pilot was killed when his F/A-18 Hornet jet crashed in an area similarly near to the Marine Corps Air Station Miramar.