A helicopter crash in Alaska in March 2021 that killed five people was attributed to pilot mishandling and inadequate training, according to a National Transportation Safety Board (NTSB) finding released Wednesday.

Among the victims was Czech billionaire Petr Kellner, founder of the PFF investment group. According to Forbes, his fortune was estimated at $17.5 billion and he was the richest man in the Czech Republic. In the Forbes list of the world’s richest people for 2020, Petr Kellner was ranked 68th along with Australian-American media magnate Rupert Murdoch.

The NTSB’s finding cited the most likely cause of the crash as the “pilot’s failure to react appropriately” when faced with reduced visibility due to snow, which ultimately led to the helicopter’s crash. It is also highlighted that the pilot’s training was inadequate, as was his periodic evaluation by the helicopter rental company.

There are also calls for the company to be inspected by the Federal Aviation Administration (FAA).