HONOLULU (AP) — The National Transportation Safety Board said Tuesday that a helicopter that crashed in Hawaii last month and killed all four people on board nose-dived from 200 feet (60 meters) above ground after it unexpectedly stopped while making a shallow left turn.

The agency’s preliminary report on the Feb. 22 crash on Kauai said U.S. Navy contractor Croman Corp. was using the Sikorsky S-61N helicopter to retrieve inert training torpedoes from the Pacific Ocean at the time.

The crew’s job involved locating a training torpedo in open waters and retrieving it using a recovery basket or cage. The helicopter then was expected to return the torpedo to the nearby Pacific Missile Range Facility using a sling load, which is cargo carried beneath a helicopter.

The agency said multiple witnesses reported the helicopter “gradually pitched nose down and impacted nose first, in a near vertical attitude.”

A fire then “incinerated much of the helicopter’s structure,” the report said.

The aircraft was returning to an ordnance recovery cage area when it crashed.

Two pilots and two crew members on board the helicopter died.