San Diego. Efforts were underway Thursday to recover the remains of five US Marines who were killed when their helicopter went down during stormy weather in the mountains outside of San Diego, the military said.

Authorities say the CH-53E Super Stallion vanished late Tuesday night while conducting flight training on its way back to Marine Corps Air Station Miramar in San Diego from Creech Air Force Base, northwest of Las Vegas.

"It is with a heavy heart and profound sadness that I share the loss of five outstanding Marines from 3d Marine Aircraft Wing and the Flying Tigers,” Maj. Gen. Michael J. Borgschulte, commander of 3rd Marine Aircraft Wing, said in a statement.

Per military policy, the names of the Marines are not released until 24 hours after all next-of-kin have been notified. The five Marines were assigned to Miramar’s Marine Heavy Helicopter Squadron 361, Marine Aircraft Group 16, 3rd Marine Aircraft Wing at Miramar.

The military is investigating the crash and Capt. Stephanie Leguizamon, spokesperson for the wing, said she had little information beyond the statement, but noted that recovery efforts were being hampered by snowfall from a historic storm that was moving out of California on Thursday.

Defense Secretary Lloyd Austin expressed his condolences and said his prayers are with the loved ones of these “brave Marines” and said the entire defense department mourns their loss.

“As the Marine Corps investigates this deadly crash, it is yet another reminder that across our nation and the world our selfless service members put their lives on the line every day to keep our country safe,” he said in a statement.

President Joe Biden said in a statement that he and first lady Jill Biden are “heartbroken” to learn of the Marines' deaths.

“Our service members represent the very best of our nation -- and these five Marines were no exception,” Biden said.

The last known contact with the Super Stallion -- the largest helicopter in the military, and designed to fly in bad weather -- was at about 11:30 p.m. Tuesday when waves of downpours and snow were hitting the region, Mike Cornette of the California Department of Forestry and Fire Protection told CBS 8 news. That location was based on a “ping” reported to a Cal Fire dispatch center.

The craft was discovered Wednesday morning near Pine Valley, an hour’s drive from San Diego.

The mountain community is at about 3,700 feet (1,127 meters) in elevation in the Cuyamaca Mountains, an area which saw as much 8 inches (20 centimeters) of accumulating snow within hours Tuesday night and early Wednesday and saw more falling Wednesday night, according to forecasters.

The area includes San Diego County’s second highest mountain, Cuyamaca Peak, at 6,512 feet (1,985 meters), and is also near the Cleveland National Forest, which covers 720 square miles (1,860 square kilometers) with much of it steep, rocky and with limited trails.

Marines fly this model in the “most challenging environments you can think of," said Timothy Loranger, a former Marine Corps aircraft mechanic and now civilian pilot and lawyer who specializes in aviation crashes.

“But even knowing that, it’s important for those making the decisions to decide whether the risk is worth it,” he said. “Here was a training mission with the crew flying back home to Miramar, so you have to ask, why fly in this weather now? Was this poor judgment or some pilot error issue? These are very important questions that have to be asked.”

A mechanical issue in bad weather would also make flying even harder. And while designed for bad weather, ice accumulation on rotor blades could disrupt the ability to create lift for the helicopter and cause it to crash, he said.

All this will be part of the investigation, Loranger said, adding that it's too early to speculate on what occurred. He said the Super Stallion has a long history of being reliable but it's not perfect.

Measuring about 99 feet (30 meters) long and weighing up to nearly 35 tons (31,638 kilograms), the CH-53E Super Stallion has been used in combat and other operations around the world for more than three decades, including in Beirut, Somalia, the Persian Gulf, Afghanistan, Iraq and Libya, according to a US Navy website.

In 2018, four Marines from Miramar died when their Super Stallion crashed near El Centro, near the California-Mexico border, during a training mission. The Marine Corps ruled out pilot error for the accident. The victims’ families later sued two companies they alleged provided a defective part that they blamed for the crash.

In 2005, a Super Stallion went down in a sandstorm in Iraq, killing 31 people on board. The accident, blamed on pilot error, was the single deadliest loss of US troops during the war.

More than 130 are in operation.

Nicknamed the “hurricane maker” because of the downwash from its three engines, the Super Stallion has a range of about 640 miles (1,029 kilometers). It was designed to carry up to 55 troops or about 16 tons (more than 13,000 kilograms) of cargo.