The U.S. National Transportation Safety Board said on Friday it is investigating an engine fire last week on a Boeing (NYSE: BA) 737-900 MAX plane operated by United Airlines at Newark Liberty International Airport in New Jersey.

The NTSB said a fire warning light indicator came on after United Airlines Flight 2376 from Fort Lauderdale, Florida landed at Newark on June 28, prompting the crew shut down one of the engines.

There was no visible smoke or fire from the engine so the airplane was towed to the gate, where maintenance personnel saw evidence of a fuel leak from the engine and heat damage and soot on the engine cases and external surfaces, the NTSB added.

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

United said it is investigating but declined further comment. Boeing referred questions to the NTSB and United.

The plane was delivered in 2020 and the LEAP-1B engine was built by French-American jet engine maker CFM International, which is co-owned by General Electric (NYSE: GE) and France’s Safran (EPA: SAF). CFM said it “is supporting the NTSB’s investigation.”