The Boeing 777 was flying from Denver to Honolulu when its right engine erupted in flames and dropped debris over northern Colorado. The plane returned to Denver with minor damage and landed safely, with all 229 passengers and 10 crew members unharmed.
Several NTSB officials from the Denver regional office began the investigation, while the investigator-in-charge for the event traveled from Washington Sunday morning.
The probe is ongoing, with the NTSB working alongside the Federal Aviation Administration, United Airlines, Boeing, Pratt & Whitney, the Air Line Pilots Association and the Independent Brotherhood of Teamsters.
The NTSB said an initial investigation of the PW4000 engine revealed the inlet and cowling separated from the engine and two fan blades were fractured. The rest of the fan blades "exhibited damage to the tips and leading edges."
The team will also examine the plane itself, photos and videos captured by passengers aboard the flight, the cockpit voice recorder and flight data, which have been transported to Washington for analysis.
Boeing has grounded the 777s equipped with PW4000 engines while the investigation continues.