Marijuana use was a contributing cause to a fatal plane crash in the Northwest Territories 18 months ago, says the Transportation Safety Board.

Two people died and two were injured when the Air Tindi plane went down in October 2011. Pilot Matthew Bromley, 28, and passenger Tim Harris, 54, died and two other passengers were seriously injured.

The wreckage of an Air Tindi Cessna 208B that crashed near Lutselk'e, N.W.T., in October 2011. The pilot and a passenger were killed. Two other passengers were seriously injured. (CBC)

The board said the amount of THC found in the pilot's bloodstream would have impaired his performance and his ability to make decisions.

"What we found, and it’s one of our findings for cause, was that the concentration of cannabinoids were sufficient to have caused impairment in pilot performance and decision making," said Jon Lee, the western regional manager for the board.

The pilot was flying under visual flight rules, but given the bad weather, it would have been safer to fly using instruments, the board said.

The report also said there was low visibility and the pilot was flying low.

Air Tindi told CBC News in an email that "they have always had a zero tolerance drug and alcohol policy."

The TSB report states that Air Tindi's drug and alcohol policy was revised following the crash. It also says following the crash, Air Tindi created a policy limiting passenger flights when the weather is not optimal.

The Air Tindi flight from Yellowknife was heading for the community of Lutselk'e when it went down about 200 kilometres east of Yellowknife, on the southeast shore of Great Slave Lake.