The flight, which left for Beijing shortly after noon, returned to Dulles about five hours later after the FBI was alerted that a child on board was allegedly being illegally taken out of the country by his mother, the official said.
Once the flight returned, the child's mother was taken into custody. She's expected to appear in federal court Friday afternoon.
The child was then reunited with its father. The child's grandmother, who was also on the plane, was removed from the flight but not detained.
A statement from United said the plane returned to Dulles "at the request of law enforcement."
The flight resumed, taking off at 7:47 p.m. ET from Dulles. It is scheduled to arrive in Beijing around 9:30 p.m. local time.