GARY | An O'Hare-bound United Airlines flight was diverted Sunday night to the Gary/Chicago International Airport, leaving 123 passengers stuck on the plane for six hours.

Passengers on the flight told Fox 32 Chicago they were not allowed to leave the plane, even though food and water ran out.

United Airlines issued a statement Monday apologizing to passengers for "this unfortunate situation" and promising to "fully investigate the incident to ensure it is not repeated."

United spokesman Jeff Kovick confirmed Monday that the plane sat in Gary for six hours. Passengers were not allowed to get off until buses arrived to take them to O'Hare.

Gary airport director Chris Curry said the airport never was asked to provide assistance to the United flight or its passengers.

United flight 1020 from Denver was scheduled to land at O'Hare International Airport at 9 p.m., Kovick said. It could not land there because of weather and other "extenuating circumstances," according to United's statement.

The Airbus 320 landed in Gary at 10:48 p.m., Kovick said.

United said the flight captain made the right decision in diverting to Gary.

Passengers said they were not allowed to use the Gary airport terminal or arrange for their own transportation to O'Hare.

The airport's terminal is closed most of the time because the airport does not now have regularly scheduled airline service.

"If we were asked to open it, especially in a case like that, we would definitely accommodate them," Curry said.

The airport has an agreement with United for diversion of planes to Gary and the airline pays a landing fee whenever it has to use the airport, Curry said.

The Gary airport tower closes at 10 p.m., so the pilot of the United flight would have activated the runway lights himself and used his instruments to land, Curry said.