A United Airlines jet headed to Rome turned around less than an hour into the flight and descended rapidly after what the airline termed an indication of possible loss of cabin pressure.
The plane returned to Newark, New Jersey, and landed safely early Thursday morning, the airline said.
United said there was no loss of pressure and passengers’ oxygen masks did not drop, but the pilots could not be certain.
Data from tracking service FlightAware showed that the Boeing 777 descended from 37,000 feet to less than 9,000 feet in eight minutes.
Pilots are trained to respond to a loss of cabin pressure by descending to an altitude that is low enough for people on board to breathe without oxygen masks.
The shortened flight had 270 passengers and 14 crew members, according to the airline.
United said it found another plane, which left Newark around 3:30 a.m. and reached Rome after 5 p.m. local time, about six hours later than the originally scheduled arrival time.