A helicopter that was carrying American coal billionaire Chris Cline earlier this month began spinning before it plunged into the ocean near the Bahamas and killed everyone on board, federal authorities in the U.S. said Tuesday.

The National Transportation Safety Board’s (NTSB) preliminary report details the July 4 accident and its aftermath but doesn’t include a cause. Cline, his daughter Kameron and three of her friends died along with a pilot and co-pilot in the crash off Cline’s private Big Grand Cay island.

The aircraft was leaving the island to take two passengers to Florida for medical treatment, the NTSB said.

A witness saw the helicopter rotate to the left three to four times, followed by a whooshing noise and the sound of an impact, the report said. The aircraft was found upside down in about five metres of water with its rotor blades separated. Investigators brought it to a secure site in the United States.

Flight and data recorders have been recovered and shipped to the NTSB in Washington for analysis, the Bahamas Air Accident Investigation Department has said.

Cline’s death led to eulogies from coal industry leaders, government officials and academics, who described him as a visionary and generous philanthropist. He accumulated a $1.8 billion US fortune from a career that he began years ago as a coal miner in southern West Virginia.

Cline’s company also owns Kameron Collieries, the company behind a coal mine in Donkin, N.S.

Cline bought Big Grand Cay in 2014.

The full investigation into the crash could take up to two years, a NTSB spokesperson said earlier this month.