The plane, a 737, landed without incident in Denver and was moved to a remote location of the airfield, the airport said. The nature of the threat was not disclosed.
In an email, an FBI spokesman in Denver said the agency was not planning to immediately comment on the situation.
One passenger tweeted from the tarmac: "Getting personal stuff off plane almost 2 hours later. Happy we are all safe thanks."
Flight 1500 landed at about 4:39 p.m., a few minutes ahead of schedule, according to Flightaware.com.
Passengers exited the plane safely, the airport said. The airport was operating normally, officials said.