Flight returns to Newark after engine problem

File photo of a United Airlines Boeing 757, like the one that developed engine trouble after takeoff Saturday from Newark.NEW: Plane returns safely to Newark airportWitnesses report fire in engineUnited flight was taking off for BerlinFBI: Piece of tire flew into Boeing 757 engine

New York (CNN) -- A Berlin-bound United Airlines flight returned Saturday night to Newark Liberty International Airport after an engine problem developed during or right after takeoff, officials said.

Eyewitnesses reported seeing flames spewing from the left engine.

A tire blew during takeoff and flew into an engine, FBI spokeswoman Barbara Woodruff said. The Federal Aviation Administration said it could not confirm that.

Flight 96 circled the airport and burned fuel for more than an hour before landing at 8:05 p.m., according to the FAA. United described it as a "mechanical issue."

The crew of the Boeing 757 reported a problem after it left New Jersey for Berlin, said FAA spokeswoman Kathleen Bergen. The engine was operating properly before it landed, she told CNN.

Eyewitness Keisha Thomas, who was traveling on the New Jersey Turnpike, said she witnessed fireballs near a wing shortly after the plane took off.

Thomas heard a loud sound, describing it as "pow, pow, pow." The plane was circling the airport, she said.

Djenaba Johnson-Jones, who lives across the river from the airport, said she heard an unusual noise and saw fire, but not smoke, coming from the aircraft's left engine.

Eyewitness Dennis Ostolaza said he heard a "propeller sound" akin to a military helicopter as the plane gained altitude after takeoff, with "black smoke and fire spitting out of the engine."

CNN's Julia Greenberg contributed to this report.

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