Guam ? The Air Force investigated the first crash of a costly B-2 stealth bomber after one plunged to the ground Saturday shortly after beginning the last flight of a four-month deployment.

Both pilots ejected safely, though one was being transferred to Tripler Army Medical Center in Honolulu to be treated for spinal compression, said Tech. Sgt. Tom Czerwinski of Pacific Air Force’s public affairs office in Hawaii.

Four of the bombers were taking off at the end of deployment to Andersen Air Force Base. At least one had taken off safely but was brought back after the crash.

A board of officers is investigating what caused the 10:30 a.m. crash, the first by a B-2 bomber, said Capt. Sheila Johnston, a spokeswoman for Air Combat Command at Langley Air Force Base in Virginia.

All 21 stealth bombers are based at Whiteman Air Force Base in Missouri, but the Air Force has rotated several of them through Guam since 2004, along with B-1 and B-52 bombers.