A fuel leak and several improperly installed parts were found inside the engine of an American Airlines plane that caught fire after the plane landed in Denver in March, according to a new report released Thursday.

The National Transportation Safety Board said one part inside the right engine of the Boeing 737-800 was loose and had been installed in an incorrect direction and that fuel was leaking from the fitting of another part that was incorrectly fastened.

The preliminary findings don’t identify the cause of the fire because the NTSB won’t reach that conclusion until after it completes its investigation sometime next year.

But former NTSB and FAA investigator Jeff Guzzetti said the problems investigators found in the engine appear to be the source of the fuel that caught fire.

“To me, it looks like improper maintenance in the right engine leading to a fuel leak,” Guzzetti said after reading the NTSB report.

Photos and videos posted online showed billowing smoke and passengers standing on the plane’s wing after it taxied to a gate at Denver International Airport. Twelve people were taken to the hospital with minor injuries.

The plane carried 172 passengers and six crew. The NTSB said gate workers extinguished the fire within a minute even before firefighters arrived and doused lingering hot spots on the plane.

Pictures included in the NTSB report show streaks on the outside of the engine from the leaking fuel, and airport video showed a trail of fluid leaking from under the right engine as the airplane taxied into the gate.

Guzzetti said he believes those streaks were probably made while the plane was flying. After it landed, he said, the fuel likely pooled inside the engine and caught fire.

The American Airlines flight had left Colorado Springs Airport and was bound for Dallas Fort Worth when it diverted to Denver on March 13 after the crew reported high engine vibrations. The plane’s right engine caught fire after it arrived at the gate.

Passengers started yelling “fire” and “smoke” shortly after the plane pulled up to the gate, and flight attendants saw smoke start to fill the cabin, according to the NTSB report.

The flight attendants tried calling the flight crew and knocking on the cockpit door, but didn’t get an answer. So the report said the flight attendants initiated the evacuation.

Passengers were able to use the jetway at the front left door and the escape slide at the right rear door as well as the left over-wing doors to evacuate. But the left rear door wouldn’t open, and maintenance personnel discovered afterward that the escape slide jammed in the door.

The faulty slide was sent to the manufacturer for further investigation.

The country has seen a recent spate of aviation disasters and close calls stoking fears about air travel, though flying remains a very safe mode of transportation.