The incident happened on a morning flight from the northern Greek city of Thessaloniki to Memmingen, near Munich, operated by Malta Air, a subsidiary of Ryanair, Europe’s largest budget carrier.

Ryanair said in a statement the flight “returned to Thessaloniki shortly after takeoff when a passenger window dislodged in-flight.”

The 61-year-old passenger, who was not identified by name, suffered neck and shoulder injuries and friction burns, according to a Greek hospital official who spoke on condition of anonymity because they were not authorized to speak publicly to the media.

It was not immediately clear if the injured passenger remained in hospital later Friday.

Passengers told Greek media that they heard a loud bang, oxygen masks dropped and the plane began to lose altitude.

One passenger, identified only as Christina, told Radio Thessaloniki that some passengers panicked and screamed and that one passenger was partially sucked out of the window.

“His whole head, neck, shoulders” were pulled out of the window, she said, adding that those seated near him pulled him back in.

“Most people had fallen asleep, we had closed our eyes. We heard a sound, I’d describe it like a tire bursting … but very loud,” she said. “We knew straight away we lost pressure because we lost altitude.”

She said there were “screams, shrieks, shouting.”

The airline has not said what caused the window to dislodge, but the U.S. National Transportation Safety Board said it was notified that the flight turned back because of “a right engine issue and cabin decompression.”

Ryanair did not immediately respond to an email request seeking comment on the engine issue.

The NTSB, the U.S. federal agency that investigates aviation and other major transportation incidents, said it was standing by to assist the investigation. It said the probe will be led by the Aircraft Accident and Incident Investigation Committee of the Republic of North Macedonia, which under international aviation rules takes the lead because the incident occurred in that country’s airspace.

The agency in North Macedonia, which borders Greece to the north, did not immediately respond to an emailed request for comment.

A series of short videos recorded from inside the plane and shared by Radio Thessaloniki showed passengers wearing oxygen masks after the cabin lost pressure. Another appeared to show the blown-out window, with a man seated nearby wearing an oxygen mask. A third video, apparently filmed after the aircraft landed, showed first responders working in the aisle.

Shye Gilad, a former airline pilot who teaches at Georgetown University’s business school in the United States, said the incident underscored the importance of keeping seatbelts fastened while seated. A rapid decompression can create a brief but powerful suction effect near a breach in the cabin before the cabin’s pressure stabilizes, he said.

“The seatbelt can help in those first few seconds. It’s a difference maker and people should keep their seatbelts fastened at all times,” Gilad said, adding that events such as Friday’s incident are “a very rare” because “it takes a lot to breach a cabin.”

The aircraft was a Boeing 737-800, which can seat up to 189 passengers. The narrow-body plane was delivered new to Ryanair in 2008, according to flight-tracking site Flightradar24.

Flight records show that the aircraft climbed past 15,000 feet (4,570 meters) about six minutes after departure and then immediately descended to about 6,000 feet (1,830 meters) “to burn fuel for 30 minutes” before returning to Thessaloniki about an hour after taking off, Flightradar24 said.

The plane landed normally and passengers returned to the terminal, and one passenger requested and received medical assistance on the ground in Thessaloniki, the airline said in a statement. A replacement aircraft was later provided to fly the passengers to Germany.