The crash was deemed unsurvivable based on a review of footage, according to Col. James Hayes, deputy commander for the 412th Test Wing at the base, calling it a “horrible tragedy.”

“We lost eight great Americans,” he said during a press briefing Monday.

The crew was a mix of uniformed military, government civilians and government contractors, Hayes said. The names of those on board will be released 24 hours following next-of-kin notification.

Boeing released a statement Monday evening saying two of those killed in the crash were employees of the aerospace giant.

“It is with great sadness that we confirm two Boeing employees were among those on board. We are in contact with their families and are offering support,” Boeing said.

The incident was reported at about 11:20 a.m. local time.

“Emergency response personnel are on scene, and officials are working to account for all personnel,” the base said.

The cause of the crash remains under investigation, a process that will likely take several months, according to Hayes.

Test missions take place multiple times a day at the base, Hayes said.

The base has closed the airfield and said all inbound planes are being diverted. It will be standing down all operations on Tuesday, Hayes said.

The Air Force and NASA conduct test flights of new and developmental aircraft at Edwards Air Force Base.

The B-52 Stratofortress is a long-range bomber first introduced in the 1950s that remains a central part of the U.S. military’s air power. Built by Boeing, the aircraft is capable of carrying both conventional and nuclear weapons over long distances and has been used in conflicts ranging from Vietnam to operations in the Iran war.