ELKO, Nev. (AP) — Federal investigators say an air ambulance that crashed after takeoff in Elko in November 2016 was repaired three times in the six weeks before it went down, killing all four people on board.

But the National Transportation Safety Board said Friday there was no evidence of mechanical malfunction or engine failure when the twin-engine aircraft bound for a Salt Lake City hospital crashed near the Elko airport.

The NTSB said earlier a witness reported the plane operated by American Medflight appeared to lose power immediately after takeoff.

Investigators said Friday toxicological tests on the pilot's remains found no evidence of alcohol or drugs.

An oil leak was repaired and fuel control unit replaced on Oct. 10. A valve was replaced on one engine three days before the Nov. 18 crash. The investigation into the cause of the accident continues.