CNN  — 

The National Transportation Safety Board is finishing up its initial examination into the deadly Southwest Airlines engine failure Thursday – but many questions remain unanswered.

The jet’s engine failed Tuesday about 20 minutes into its flight from New York City to Dallas. Debris from the engine blew out a window, causing passenger Jennifer Riordan to almost get sucked out of the plane.

Passengers dragged her back into the cabin, but she died at a Philadelphia hospital after the plane made an emergency landing.

Passenger Jennifer Riordan died in the Southwest Airlines incident Tuesday.

Check and travel voucher

In a letter to passengers, the airline expressed “sincere apologies” for the fatal incident and included the check, said Kamau Siwatu, who was aboard the flight. He was one of at least three passengers who said they received the letter.

“We value you as our customer and hope you will allow us another opportunity to restore your confidence in Southwest as the airline you can count on for your travel needs,” Siwatu’s letter said. “In this spirit, we are sending you a check in the amount of $5,000 to cover any of your immediate financial needs.”

In addition to the money, passengers will receive a $1,000 travel voucher, the letter said.

It’s not unusual for airlines to give passengers money in such situations, according to CNN transportation analyst Mary Schiavo. “It gets money in the hands of people that need it for counseling or something,” Schiavo said.

CNN has reached out to Southwest Airlines for comment, but has not heard back.

Search for answers

The plane’s engine had undergone a visual inspection two days before, the airline said.

The National Transportation Safety Board is finishing up its initial examination into the deadly engine failure – but many questions remain unanswered.

Investigators can’t explain with certainty why the left engine of Southwest Airlines Flight 1380 malfunctioned when the plane was reaching over 32,000 feet. It’s too soon to determine what happened, said National Transportation Safety Board Chairman Robert Sumwalt.

“I know people would want answers right away. We would do a very methodical investigation,” Sumwalt told reporters Wednesday in Philadelphia. “Right now, we just want to document everything that we can.”

They’ve interviewed the pilots, examined the damaged engine and collected video footage from passengers who lived through those 22 minutes of chaos. They recovered the airplane’s cockpit voice recorder and flight data recorder. And they are still tracking down debris from the engine.

The NTSB will look into how an interior crack on a fan blade could have led to the engine failure. Sumwalt said it was “certainly not detectable from looking at it from the outside.”

“My hat is off to them, they behaved in a manner their training called for,” he added.

CNN’s Sheena Jones contributed to this report.