The FBI is involved in the investigation.
United flight 2092 departed from O'Hare airport and was headed to New York LaGuardia airport Saturday morning.
The flight was diverted to Pittsburgh "to address a potential security concern," United said.
The pilots reported hearing a "sequential beeping" onboard, ABC News reported.
Passengers evacuated the plane on slides.
No injuries were reported. There were six crew members and 159 passengers on board.
Alleghany County police said they completed their search of the plane, and did not find anything.
The flight was able to take back off for New York.
ABC News contributed to this report.