One Marine died after an MV-22 Osprey aircraft crashed in a "hard-landing mishap" at Bellows Air Force Station in Wai­ma­nalo during a routine training exercise late Sunday morning.

Twenty-one others were sent to three Oahu hospitals with varying degrees of injury following the 11:40 a.m. incident. Twenty-one Marines and one Navy corpsman assigned to the unit were aboard the aircraft, according to the Associated Press.

The Marines are attached to the 15th Marine Expeditionary Unit based at Marine Corps Base Camp Pendleton in Cali­for­nia.

The cause of the incident is under investigation, Marine officials said.

Capt. Alex Lim, public affairs officer for U.S. Marine Corps Forces Pacific, said the unit was conducting routine sustainment training when the hard landing occurred.

Ewa Beach resident Desi­ree Fau­mui was at a family gathering on the public side of Bellows when the group noticed three of the aircraft hovering at a low elevation, circling between just offshore and within the gated area of the Bellows base.

After about 15 minutes of circling, the three appeared to descend inside the base behind a line of trees, and moments later a large dust cloud appeared, Fau­mui said, noting that the cloud appeared to be a soil-like red.

"And then only two came up," she said. A minute later her group saw gray smoke that was followed by a plume of thick, black smoke.

Windward residents in the Kai­lua, Keolu Hills and Enchanted Lake areas reported seeing smoke rising from the Bellows area.

Lim said the hard landing took place in a flat landing zone area known as "LZ Gull," about 1,000-2,000 yards from the entrance to the Air Force station, which is next to the public beach area.

The Osprey was not participating in an exercise connected to the U.S. Pacific Command Amphibious Leaders Symposium. Marine Corps Forces Pacific is hosting 23 foreign nations at the symposium, which started Sunday and is scheduled to wrap up Thursday in Hawaii. Senior military leaders of allied and partner nations are expected to attend.

The unit has been in Hawaii for about a week, preparing for a seven-month deployment to the Pacific Command and Central Command areas of operation, according to the Marine Corps’ official website.

The distinctive MV-22 Osprey tilt-rotor aircraft has had a checkered past.

Built by Boeing Co. and Bell, a unit of Textron Inc., the Osprey was nearly scrapped after a history of mechanical failures and two test crashes that killed 23 Marines in 2000, according to the Associated Press.

Since then the aircraft has been deployed to Iraq and Af­ghani­stan. Some Ospreys are also helping with earthquake relief efforts in Nepal.

The 57-foot-long aircraft has twin 38-foot propellers that allow it to take off like a helicopter and fly like a conventional plane with the blades rotated forward. Ospreys can carry 24 Marines twice as fast and five times as far as previous helicopters.

The aircraft made its first appearance in Hawaii nearly two years ago.

Two squadrons of Ospreys, a total of 24 aircraft, are scheduled to be based at Kane­ohe Bay in fiscal 2015 and 2016.