A fuel leak may have been the cause of an engine fire that occurred on a United Airlines Boeing 737 MAX 9 aircraft in Newark last week, the US National Transportation Safety Board (NTSB) states in a preliminary report.

The aircraft, operating as flight UA 2376, originated in Fort Lauderdale, Florida, and arrived at Newark Liberty International Airport at 10:19 local time on June 28 after two and a half hours of flight. According to the NTSB report, released July 7, none of the five crew and 179 passengers were injured. It continues to investigate the incident.

The report states that “According to the flight crew, during taxi-in, they observed a No. 1 engine fire warning indication…The flight crew shut down the No. 1 engine, discharged one fire bottle, and the fire warning indication ceased.”

“There was no visible smoke or fire emanating from the No. 1 engine, so the airplane was towed to the gate. Upon gate arrival, maintenance personnel observed evidence of a fuel leak from the No. 1 engine,” NTSB adds. ”The No. 1 engine thrust reverser doors were opened for visual inspection and heat damage and sooting were observed on the engine cases and external surfaces.”

The incident occurred as the Chicago-based carrier was in the throes of dealing with an operational meltdown that hit its east-coast hub Newark hardest following sustained inclement weather over the course of several days. United Airlines chief executive Scott Kirby blamed the FAA’s air traffic controller shortage for many of the problems, but US Secretary of Transportation Pete Buttigieg, under whose leadership the FAA sits, fired back to say that staffing issues contributed to the chaos, but only marginally.

Passengers exited from the plane in a relatively normal fashion in Newark, according to the Federal Aviation Administration (FAA). The FAA said after the plane landed the flight crew activated the engine fire extinguishers as a precaution.

The aircraft holds the registration N37516 and was delivered to United at the end of December 2020, according to Cirium fleets’ data. According to Cirium, the airline has 66 of the same type of aircraft in service, with 13 more on order.

This isn’t the first time that this specific model has had functional issues. Boeing unveiled its 737 MAX in 2015, and since its Federal Aviation Authority (FAA) approval in 2017, it became one of the most widely used aircraft in the world. A year later it had its first crash. In October 2018 a 737 MAX operated by Indonesian airline Lion Air crashed shortly after take off, killing all 189 on board.

Five months later, in March 2019, a second 737 MAX, operated by Ethiopian Airlines, crashed again shortly after takeoff, killing all 157 on board. Three days later the planes were grounded by the FAA. It later emerged that Boeing staff, in internal messages, were cavalier about FAA regulations and critical of the MAX's design.

Faults were discovered in the aircraft's MCAS or Maneuvering Characteristics Augmentation System: the MCAS was found in both the Lion Air and Ethiopian Air crashes to have mistakenly pointed the nose down toward the ground, and the pilots were unable to override it.

In 2021, Boeing agreed to pay $2.5 billion in fines in a deferred prosecution agreement with the U.S. Justice Department to settle charges the company hid critical information about the Max from regulators and the public.

Boeing spent billions overhauling the systems and the planes returned to global skies in the fall of 2020, after being grounded for 20 months, the longest of such an action in aviation history.