Marijuana use by the pilot was a factor in a deadly plane crash in the Northwest Territories in 2011, the Transportation Safety Board (TSB) said in a report Wednesday.

The pilot and a passenger were killed when the Cessna, operated by Air Tindi, went down while travelling from Yellowknife to Lutsel K'e, N.W.T., on Oct. 4. Two other passengers were seriously injured.

"Toxicology testing revealed that concentrations of cannabinoids found in the pilot's bloodstream were sufficient to have impaired pilot performance and decision-making during the flight," a statement from the TSB said.

Since the crash, the operator has launched random drug and alcohol testing for its employees.

The pilot was found to have been flying at a low altitude before the crash, and visibility was sub-par.