Five U.S. Marines who were aboard a military helicopter that went down in the mountains near San Diego were found dead, authorities said on Thursday.

The CH-53E Super Stallion helicopter was reported missing Tuesday night during a "routine training flight" from Creech Air Force Base, northwest of Las Vegas, to Marine Corps Air Station Miramar in San Diego, according to the 3rd Marine Aircraft Wing.

"It is with a heavy heart and a profound sadness that I share the loss of five outstanding Marines," Maj. Gen. Michael Borgschulte, commanding general of the 3rd Marine Aircraft Wing, said in a statement. "To the families of our fallen Marines, we send our deepest condolences and commit to ensuring your support and care during this incredibly difficult time."

The aircraft wing said efforts to recover the remains of the Marines and equipment have begun and an investigation is underway. The names of the Marines will not be released until 24 hours after all next-of-kin notifications are completed. The cause of the crash remains unknown.

Learn more: Graphics and maps show flight, crash site.

President Joe Biden in a statement said he and first lady Jill Biden are “heartbroken” at the death of the Marines.

“As the Department of Defense continues to assess what occurred, we extend our deepest condolences to their families, their squadron, and the U.S. Marine Corps as we grieve the loss of five of our nation’s finest warriors," Biden said. “Our service members represent the very best of our nation and these five Marines were no exception."

Helicopter discovered in Pine Valley amid heavy snowfall

Local, state and federal authorities began a search early Wednesday and discovered the aircraft in Pine Valley, California, about 45 miles east of San Diego, just after 9 a.m. The condition of the aircraft has not been released nor the time the Marines departed from Nevada or when they were scheduled to land at the base in San Diego.

The last known contact with the helicopter was at about 11:30 p.m. Tuesday, Cal Fire’s spokesperson Capt. Mike Cornette told CBS 8 news. Its location was based on a "ping" reported to a Cal Fire dispatch center.

Harsh conditions and heavy snow from a historic storm that hit California this week made it challenging for rescue crews to access the area, said Cal Fire and the San Diego County Fire Protection District. The storm pummeled the region with record-setting rainfall beginning over the weekend and lasting through Tuesday, when the helicopter was scheduled to arrive in San Diego. The atmospheric river storm also brought heavy snow across the region's mountains.

The San Diego County Sheriff's Department along with the Civil Air Patrol, the U.S. Border Patrol Search, Trauma and Rescue (BORSTAR) Unit, Cal Fire San Diego and the U.S. Forest Service were involved in the sweeping search effort. Jeeps and drones were used to manage the difficult terrain.

CH-53E Super Stallion has been involved in several deadly incidents

The CH-53E Super Stallion is one of the largest helicopters used in the U.S. military. It is described as a "heavy-lift helicopter" capable of transporting troops, equipment and supplies from ships to shore and can also launch amphibious assaults, according to the Naval Air Systems Command website.

The aircraft, made to withstand harsh conditions, such as moving over rugged terrain in bad weather, has been involved in several deadly incidents over the last 20 years.

In April 2018, four crew members were killed when the CH-53E Super Stallion they were on went down a few miles from the U.S.-Mexico border. The helicopter was also with the 3rd Marine Aircraft Wing out of the Miramar air station in San Diego.

Two years earlier, 12 Marines died when two Super Stallion helicopters collided off the coast of Oahu in Hawaii. In 2005, 31 people died when a CH-53E helicopter crashed in Iraq during a sandstorm.

Contributing: Associated Press; N'dea Yancey-Bragg, Thao Nguyen, USA TODAY