Federal Aviation Administration spokesman Ian Gregor said Flight 1181 from Los Angeles International Airport to Dulles International Airport landed "without incident" in Ontario. The Boeing 757 took off at 8:25 a.m. and Gregor said it landed around 9 a.m.
The plane was met by Ontario International Airport Rescue and Fire personnel on the tarmac.
The airline did not immediately specify what triggered the emergency landing, but a United Airline spokesperson said all 199 passengers including seven crew members were evacuated safely. There were no reports of injuries.
Passengers took to social media, saying smoke in the cabin forced the landing. A witness captured video of firefighters dousing the plane's engine shortly after it landed safely on the runway.
Photo via twitter.com/mitchellh
"They can't deplane us until the fire is out, it's not in the cabin. Cabin has smoke in it though. It was smoky mid-flight. Again, not fun," tweeted passenger Mitchell Hashimoto, who later deleted the post, citing unwanted media attention.
After getting off the plane, passengers had to wait on the tarmac to board buses. Officials said passengers will be kept in Terminal 2 before boarding another United jet bound for Washington, D.C.
United Airlines said its maintenance team is looking over the aircraft to determine the cause of the smoke.