Foreign object damage is what caused a 2016 crash in which an AV-8B Harrier went down in the Atlantic Ocean off North Carolina, an investigation found.

The pilot, from Marine Attack Squadron 542 in Cherry Point, North Carolina, safely ejected uninjured on May 6, 2016.

The foreign object that caused the Harrier’s engine to tear itself apart was likely a “28 threads-per-inch fastener with a 5/16th inch double hexagon bolt head,” according to a redacted copy of the crash investigation, which Marine Corps Times obtained through a Freedom of Information Act request.

The bolt is used on Harriers and other aircraft. Investigators determined that the fastener probably did not come from the runway at Marine Corps Air Station Cherry Point or Wilmington International Airport in North Carolina. It is more likely that the fastener had been lodged in a door near the engine’s intake.

At that time, the pilot had flown a total of 438.4 hours, including 177.9 hours in Harriers, the investigation found. He had last flown 10 days earlier, and had received a perfect score on his emergency procedures exam two days before the mishap.

During the flight, the pilot heard a “thump” while accelerating, according to the investigation. The plane was shaking and he did not get any warning lights, so he continued with the mission, saying over the radio, “I don’t know what that was, but it scared me.”

The pilot heard the thump a second time, but again all seemed normal.

But when he heard the noise for the third time, “it was more violent and shook the aircraft,” he told investigators.

The pilot pushed the throttle up, but the engine was dead. He tried to restart the engine but nothing happened as his plane fell toward the ocean.

With his engine refusing to restart and his plane’s generator offline, the pilot noticed he was getting closer to the water, “So I made the decision to pull the ejection handle.”

He punched out 6 minutes and 5 seconds after hearing the first thump.

The investigation determined that the noise the pilot heard was the impending engine failure. Once the Harrier’s compressor stalled and the aircraft lost thrust, the crash became inevitable.

Both the pilot and the maintainers were cleared of violating any orders or standards that could have led to the crash.

“At the time of this report’s submission, the AV-8B Fleet Support Team was able to determine FOD [foreign object damage] caused the catastrophic engine failure, but not the exact origin of the FOD,” the investigating officer wrote in the report. “I recommend further EI [engineering investigation] analysis to answer this question as it may lead to changes in maintenance inspection, pilot preflight inspection, and foreign airfield operations procedures.”