Three U.S. Marines were killed and five others seriously injured on Sunday in an aircraft crash during a training exercise in Australia. Twenty-three Marines were on board during the crash on Melville Island at around 9:30 a.m. local time. The cause of the crash is “under investigation,” according to a Sunday statement from the Marine Rotational Force—Darwin. CNN military analyst Cedric Leighton said that it’s still too early to tell if the disaster was due to a mechanical failure or not, but some Marines are still being treated on-site. “We are working incredibly hard and as fast as we can to make sure we can get people to treatment,” Northern Territory Chief Natasha Fyles said. All 20 survivors were taken to hospital and by Monday local time, eight Marines remained, Fyles said in a press conference. Of the five critically injured, one underwent emergency surgery. “It’s ... a credit to everyone involved that we were able to get 20 patients from an extremely remote location on an island into our tertiary hospital within a matter of hours,” Fyles said.