DENVER — The government on Wednesday began investigating how a United Airlines jetliner hit severe turbulence on a cross-country flight over Missouri, injuring at least 22 and jolting one woman out of her seat so forcefully that she left a crack when she hit the side of the cabin, authorities and a witness said.

DENVER — The government on Wednesday began investigating how a United Airlines jetliner hit severe turbulence on a cross-country flight over Missouri, injuring at least 22 and jolting one woman out of her seat so forcefully that she left a crack when she hit the side of the cabin, authorities and a witness said.

The Tuesday flight was the airline's third this year during which passengers were hurt because of turbulence.

The flight originated at Dulles International Airport near Washington, D.C., and was headed to Los Angeles. It was diverted to Denver International Airport, where it landed safely around 7:45 p.m. and was met by medical crews.

Flight 967 was about 90 miles east-southeast of Kansas City, Mo., at an altitude of about 34,000 feet when it hit the heavy turbulence. It was carrying 255 passengers and 10 crew members. Federal Aviation Administration spokesman Ian Gregor said the FAA had issued a warning to the aircraft about thunderstorms before it hit the turbulence.

There were conflicting reports on injuries. Denver Health medical center said a total of 21 people were taken to five hospitals, and one other person was treated at the airport. By Wednesday afternoon, all 21 people, including a 12-year-old, had been released from the hospital.

Many of them had neck and back injuries.

The FAA said earlier that 30 were injured, one critically, but Denver Health officials said they had no record of anyone in critical condition.

United spokesman Mike Trevino said four flight attendants were among the injured, but he had no other details. Trevino said some of the passengers were placed on another plane with a new crew and left Denver on Tuesday night. FAA inspectors found no obvious damage to the plane's exterior.

The National Transportation Safety Board is also investigating the incident, board spokeswoman Bridget Serchak said. FAA spokesman Mike Fergus said the incident would be a "front-burner item" for both the FAA and the NTSB.

Passenger Kaoma Bechaz, 19, of Melbourne, Australia, said the flight was delayed for about an hour in Washington because of bad weather.

The flight had been quite smooth until a few minutes before the severe turbulence, and then, "It was just a little shaky like you feel on most flights," she said.

She said the seat belt sign was on, and the flight attendants were seated. She said the turbulence seemed to catch everyone off guard, including the plane's captain. She said a woman behind her hit her head on the wall around the window, putting a crack in the side wall. She said a girl flew up and hit her head on the roof and appeared to have back pain.

"The whole plane felt like it was dropping. It was a bit chaotic at the time. The oxygen masks dropped. Loose items flew around the plane. A lot of people got drenched with other people's drinks, because we had just got our drinks and snacks. I was covered in tomato juice," said Bechaz, who was wearing a seat belt and wasn't thrown.

The crew decided land the Boeing 777 in Denver to tend to the injured, United Airlines spokeswoman Megan McCarthy said.

Officials said earlier the turbulence was in Kansas. Fergus said the confusion may have arisen because the plane was under the jurisdiction of controllers in Kansas City, which aviation officials often refer to as "Kansas airspace," even though the plane was over Missouri.

The National Weather Service said a line of strong thunderstorms extended from the middle of Missouri through the middle of Kansas on Tuesday evening. Thunderstorms, with updrafts of up to 100 mph, can cause bumpy rides for airplanes as they pass from an area of calm air to churning air, much like a speed boat hitting choppy waters, said Chad Gimmestad, a weather service meteorologist in Boulder.

Gimmestad said forecasters can't predict where those bumps will occur, so airliners generally try to fly around such storms.

The website FlightAware.com, which uses information from the FAA to track the path of aircraft, shows the United jet flew south of a storm in Missouri and Kansas.

The plane crossed the southeast corner of Kansas state and was in northern Oklahoma, about 90 miles north of Oklahoma City, when it turned northwest toward Denver.

In February, about 20 people were hurt when a United flight with 263 people onboard experienced turbulence halfway through a 13-hour trip from Washington, D.C., to Tokyo.

In May, 10 people suffered injuries, including broken bones, on a United flight that hit severe turbulence over the Atlantic Ocean on its way from London to Los Angeles. The Boeing 777 was diverted to Montreal.

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Associated Press writers Judith Kohler, Thomas Peipert and Colleen Slevin in Denver; Joan Lowy in Washington; and Denise Petski in Los Angeles contributed to this report.