American Airlines Flight 55, en route to Chicago, was diverted for security reasons, said Arlene Murray, New England regional spokeswoman for the Federal Aviation Administration. She declined to elaborate.
The Boeing 767 jet left Manchester at 10:45 a.m. local time, and was scheduled to land in Chicago at 1 p.m. CDT. The plane is now on the ground in Bangor, CBS News reports.
The threat was described as "not vague," but "kind of specific," according to CBS News reports. CBS News has learned that this was not a simple "unruly passenger" situation.
Police confirmed that there was an incident at the airport but provided no details. Federal agents were at the airport.
FBI spokeswoman Gail Marcinkiewicz declined to discuss the nature of the problem.
"We're responding to determine the cause of the diversion. It's going to take a number of hours for passengers to be interviewed," Marcinkiewicz said from Boston.
An American Airlines spokesman in Dallas said the flight, which was carrying 167 passengers and 12 crew members, was forced to land in Maine at the request of the Transportation Security Administration because of an unspecified "security concern."
"The plane has landed safely and passengers are being de-planed while TSA canines search the airplane. Responders are on the scene and the FBI will be interviewing the passengers and crew to further assess the situation," a TSA statement said.
Airline spokesman John Hotard added that the flight will continue to Chicago once a new crew is in place, as the delay would have forced the current crew to work beyond their required hours.
Hotard declined to say whether any passengers were removed from the flight.
The Bangor airport is a popular place for international flights to touch down because of mechanical problems, unruly passengers or security threats.
Also on Friday, a US Airways jet flying to Charlotte, N.C., was diverted to Oklahoma City after an air marshal subdued a passenger involved in an incident with a flight attendant, an airline spokesman said.
Law enforcement officers were questioning the passenger after the jet landed at Will Rogers World Airport, said Morgan Durrant, a U.S. Airways spokesman. He did not disclose the nature of the problem between the passenger and the flight attendant.
The Airbus A321, which left Phoenix at 6:15 a.m., was to resume its flight to Charlotte, Durrant said. There were 141 people, including crew members, on board.