PHILADELPHIA >> The investigation into a deadly engine failure on a Southwest jet is focusing on whether wear and tear caused a fan blade to snap off, triggering a catastrophic chain of events that killed a passenger and broke a string of eight years without a fatal accident involving a U.S. airliner.

From investigators’ initial findings, the accident appears remarkably similar to a failure on another Southwest plane two years ago — an event that led the engine manufacturer and regulators to push for ultrasonic inspections of fan blades on engines like the one that blew apart at 32,500 feet over Pennsylvania on Tuesday.

When investigators from the National Transportation Safety Board examined the broken engine in Philadelphia just hours after it made an emergency landing, they immediately saw that one of the left engine’s 24 fan blades was missing.

“This fan blade was broken right at the hub, and our preliminary examination of this was there is evidence of metal fatigue where the blade separated,” said NTSB chairman Robert Sumwalt.

Metal fatigue is a weakening of metal from repeated use and involves microscopic cracks. It can occur in fan blades, the aluminum skin on most planes, or other metal parts.

Investigators will focus on whether the fan blade broke off at cruising speed — around 500 mph — and started an “uncontained” engine failure that sent debris flying like shrapnel into the plane, where it broke a window.

A woman sitting near the window was sucked partially out of the plane before other passengers managed to pull her back in.

A registered nurse and emergency medical technician on board jumped in to try to save the gravely injured woman. But Jennifer Riordan, a Wells Fargo bank executive and mother of two from Albuquerque, New Mexico, died later. Seven other victims suffered minor injuries.

The pilots of the twin-engine Boeing 737 bound from New York to Dallas with 149 people aboard made an abrupt turn toward Philadelphia and began a rapid descent after the engine blew. Oxygen masks dropped from the ceiling, and passengers prayed and braced for impact.

“We heard a loud noise and the plane started shaking like nothing I’ve ever experienced before. It sounded like the plane was coming apart, and I think we pretty quickly figured out that something happened with the engine,” retired nurse Peggy Phillips told WFAA-TV in Dallas.

She said they started losing altitude and the masks came down, and “basically I think all of us thought this might be it.”

Then she heard commotion a few rows behind her.

“It was a lot of chaos back there — a lot of really upset people and a lot of noise, and a big rush of air, a big whoosh of air,” Phillips said.

After a flight attendant asked if anyone knew CPR, Phillips and an EMT laid the woman down and performed CPR for about 20 minutes until the plane was on the ground.

“If you can possibly imagine going through the window of an airplane at about 600 mph and hitting either the fuselage or the wing with your body, with your face, then I think I can probably tell you there was significant trauma,” Phillips said.

Photos of the plane on the tarmac showed a missing window and a chunk gone from the left engine, including part of its cover. A piece of the covering was later found in Bernville, Pennsylvania, about 70 miles (112 kilometers) west of Philadelphia, Sumwalt said.

As a precaution, Southwest said it will inspect similar engines in its fleet over the next 30 days. CEO Gary Kelly said there were no problems with the plane or its engine when it was inspected on Sunday.

Passengers praised one of the pilots, Tammie Jo Shults, for her cool-headed handling of the emergency. The former Navy pilot was at the controls when the plane landed, according to her husband, Dean Shults. She walked through the aisle and talked with passengers to make sure they were OK after the aircraft touched down.

“She has nerves of steel. That lady, I applaud her,” said Alfred Tumlinson, of Corpus Christi, Texas. “I’m going to send her a Christmas card, I’m going to tell you that, with a gift certificate for getting me on the ground. She was awesome.”

The jet’s CFM56-7B engines were made by CFM International, jointly owned by General Electric and Safran Aircraft Engines of France. CFM said in a statement that the CFM56-7B has had “an outstanding safety and reliability record” since its debut in 1997.

In 2016, a Southwest Boeing 737-700 blew an engine as it flew from New Orleans to Orlando, Florida, and shrapnel tore a 5-by-16-inch hole just above the wing. The plane landed safely. The NTSB said a fan blade had broken off, apparently because of metal fatigue.

Last year, the engine maker and the Federal Aviation Administration instructed airlines to make ultrasonic inspections of the fan blades of engines like those in Tuesday’s accident.

A Southwest spokeswoman said the engine that failed was not covered by that directive, but the airline announced it would speed up ultrasonic inspections of its CFM56-series engines anyway.