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New York (CNN) -- A Berlin-bound United Airlines flight returned Saturday night to Newark Liberty International Airport after a problem developed in the left engine, officials said.
Eyewitnesses reported seeing flames spewing from engine right after the plane took off.
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, with 173 passengers and crew, circled the airport and burned fuel 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."
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."
The flight left the gate at 5:53 p.m.; witnesses reported seeing the engine flames shortly before 6:30 p.m.
CNN's Julia Greenberg, Ross Levitt and Jason Kessler contributed to this report.