HOULTON, Maine — Fatigue and an unauthorized nighttime approach below standard minimum altitude were factors in a fatal plane crash near the airport here in August 2015, federal investigators have concluded.

Bakary Doucoure, 30,of Le Blanc, France, was killed when the twin-engine Piper PA-44-180 he was piloting crashed at 2 a.m. Aug. 27, 2015, into some trees and landed in a pond about 2.5 nautical miles short of the Houlton International Airport. A wing and other debris were found on the ground and in trees surrounding the area.

Investigators from the National Transportation Safety Board concluded that the pilot’s descent that night was below the published minimum descent altitude for a nighttime instrument landing without visual contact with the runway.

“The pilot’s acute fatigue and his decision to attempt an instrument approach procedure that was not authorized at night” also were contributing factors, according to the NTSB final accident report, which was released Nov. 29.

At the time, Doucoure was on the third leg of a flight from Keflavik International Airport in Iceland and on his way to deliver the plane to a customer in Florida.

Radar data indicated that the pilot conducted the entire approach toward the Houlton runway at altitudes 300 feet to 700 feet lower than the instrument approach procedure authorized, according to the report. Tracking data recovered from an onboard GPS unit depicted the airplane making “S” turns back and forth across the final approach course before the track stops to the right of the extended runway centerline in the vicinity of the accident site.

The investigators said the distribution of the wreckage was consistent with a controlled flight into the terrain and there was no evidence of any mechanical failure. Toxicology tests on the pilot also came up negative for drugs and alcohol.

But the reported 300-foot ceiling at the time of the accident was well below the approach’s published 700-foot ceiling minimum for planning purposes, and the approach was not authorized at night, the report stated.

Benjamin Torres, the fixed base operator at the Houlton airport, said Thursday that he was “shocked” to learn the pilot was flying that low.

“He was much lower than he should have been,” Torres said. “I am really surprised to hear that he was so low.”

Investigators also determined that the plane crashed in Houlton 21.6 hours after it left Iceland. During that time Doucoure was in the air 16.6 hours, according to the NTSB report. The plane made stops in Greenland and Labrador before heading for Houlton.

Required operational duties such as fueling, flight planning and touching base with customs officials “would have precluded or limited any opportunity for restorative sleep,” the investigators wrote.

U.S. Customs and Border Patrol agents who were not named in the report and were awaiting the airplane’s arrival in Houlton heard it approach and described the engine noise to NTSB investigators as “smooth and continuous, until the sounds of impact were heard.”

One agent said that the runway lights were “never illuminated,” and that it was foggy, “misty,” and that visibility was poor.

Torres said that the lights at the airport are never left on at night because the airport has an automated system that allows pilots to activate the runway lights themselves by clicking communication equipment in the cockpit.

Torres said he also saw documents noting that Doucoure’s sleep time had been “limited,” saying that fatigue can definitely have an impact on the way a pilot makes decisions.

“This is a tragedy all around,” he said. “He wasn’t a very old individual.”