Eight people, including two Boeing employees, have been killed after a US Air Force B-52 bomber crashed immediately after takeoff from Edwards Air Force Base in southern California.
The incident occurred at 11:20 local time (19:20 GMT) on Monday while the aircraft had been on a routine test mission. The crash sent a huge plume of black smoke into the air that could be seen for miles.
"Today, Edwards Air Force Base experienced a terrible tragedy, and we lost eight great Americans," Col James Hayes said, describing them as a "mixed crew of military, government civilians and government contractors".
The base earlier said that initial indications were that the crash "was not survivable".
The crew's next of kin were being notified and will be named 24 hours after that, Hayes said in an afternoon briefing.
The crash was "totally contained" within the Edwards Air Force Base on the runway, Hayes said, and the base has temporarily grounded operations.
The B-52 had been supporting the base's radar modernisation programme, he said, and crashed immediately after takeoff and burst into flames.
After reviewing initial footage, the incident had been deemed "an unrecoverable crash and unsurvivable", Hayes said.
No cause has yet been determined and will not be until after a series of investigations, which could take up to 30 days. Further cause analysis investigations could take more than six months, Hayes said.
Boeing confirmed in a separate statement that two of its employees had been among those onboard and said the company was in contact with their families. The aerospace giant declined to comment further when contacted by the BBC.
California Governor Gavin Newsom called the crash "a tragic incident" and offered condolences to the victims' families and Edwards Air Force Base community in a post on X.
Aerial footage showed a charred, smoking landscape where the crash occurred.
The base said in an earlier update that all inbound aircraft had been diverted and all non-commercial visitor passes suspended "until further notice to allow the installation to focus entirely on emergency response operations."
The Boeing B-52 Stratofortress has been used by the US military since the 1950s. It is nicknamed "the Buff", which is partly short for "Big Ugly Fat".
The B-52 is a long-range strategic bomber that has been participating in bombing raids during the US and Israel's war in Iran.
Capable of flying at up to 50,000ft - in comparison to commercial airliners, which fly at around 35,000ft - the colossal bomber's 70,000lb (32,000kg) payload can include hundreds of conventional bombs and 32 nuclear cruise missiles.
The aircraft can refuel mid-flight, giving it a potentially unlimited strike range. This created a "nuclear umbrella" for the US during the Cold War, back in the era of mutually assured destruction in the mid-20th Century.
The planes typically carry a crew of five - an aircraft commander, pilot, radar navigator, navigator and electronic warfare officer.
Edwards Base is located around 100 miles (160km) north of Los Angeles in the Mojave Desert.
"Absolutely heartbreaking. God bless the 8 crew members on board," US Representative Vince Fong, whose California congressional district includes part of the base, wrote on X.
Congressman Jay Obernolte, whose district also includes part of the base, posted on Facebook that he and his wife were "praying for everyone involved in today's B-52 crash at Edwards Air Force Base, especially the crew, their families, and the first responders on scene".
Michigan Congresswoman Lisa McClain was also wrote about the crash, posting on X: "My prayers are with everyone involved in the B-52 crash at Edwards Air Force Base this afternoon."
She thanked emergency crews and added: "Our service members carry the weight of this nation's defense every single day. We are with them."
