DENVER -- A pressurization problem in the cabin forced United Airlines pilots to return to Denver instead of continuing a flight to Seattle Sunday afternoon.

United Flight 591 left Denver International Airport for Seattle just before 4 p.m. Pilots turned the plane around about twenty minutes into the flight.

The plane landed safely in Denver just after 5 p.m., but six passengers opted to go to the hospital. A United spokeswoman didn’t have information on conditions or ages of those taken to the hospital.

The Boeing 737 had 121 passengers, three flight attendants and two pilots on board.

The plane was taken out of service until mechanics finish a complete maintenance check of the plane. United officials had no word on the cause of the pressurization problem on Sunday evening.

The pressurization issue did not cause the oxygen masks to deploy, but United Airlines spokeswoman Megan McCarthy, said the situation was not one that would trigger the masks to drop.