CANBERRA, Australia (AP) — A pilot was confused and disoriented from inhaling poisonous fumes in the moments before his seaplane plunged into a river near Sydney in 2017, killing him and his five British passengers, a crash investigation found Friday.

Canadian pilot Gareth Morgan, 44, and the passengers had elevated levels of carbon monoxide in their blood. Investigators found cracks in the exhaust system of the DHC-2 Beaver plane that was built in 1963 and bolts missing from a firewall that would have allowed carbon monoxide to leak from the engine bay into the cabin.

“The pilot would have almost certainly experienced effects such as confusion, visual disturbance and disorientation,” Australian Transport Safety Bureau Chief Commissioner Greg Hood said in a statement. “Consequently, the investigation found that it was likely that this significantly degraded the pilot’s ability to safely operate the aircraft."

The crash occurred during a joy flight on New Year’s Eve when the plane flew low into Jerusalem Bay, which is surrounded by steep terrain, and crashed into the Hawkesbury River. The crash investigator said the plane likely stalled while turning in the bay before crashing.

The passengers were catering giant Compass Group chief executive Richard Cousins, his fiancee Emma Bowden, her 11-year-old daughter, Heather Bowden-Page, and his two sons William, 25, and Edward, 23.

The daughter’s father is suing the aircraft’s owner, Sydney Seaplanes, for the tragedy in the New South Wales state Supreme Court.

Sydney Seaplanes has since installed advanced carbon monoxide warning systems on their planes.

Sydney Seaplanes blames a maintenance company approved by the Australian aviation safety regulator for the tragedy.