Passengers were thrown from their seats, drinks and loose items flew through the cabin and oxygen masks dropped from overhead when a United Airlines jetliner took a harrowing drop amid severe turbulence on a cross-country flight.

One passenger was left drenched in tomato juice in the turbulence that hit just after flight attendants finished beverage service. A witness said woman was thrown out of her seat and hit her head against the ceiling. Another hit her head on the wall, leaving a crack near a window.

"Everyone was quite panicked," 19-year-old Kaoma Bechaz said Wednesday, one day after United Flight 967 hit turbulence over southwest Missouri. At least 22 people were hurt, but none of the injuries appeared serious.

"The whole plane felt like it was dropping. It was a bit chaotic at the time," Bechaz said.

Federal authorities are investigating the incident and said it was a top priority to figure out what happened. It marked the third time passengers have been injured in turbulence on United flights in recent months.

The flight had left Dulles International Airport near Washington, D.C., with 255 passengers and 10 crew members and was headed for Los Angeles. It was at about 34,000 feet when it hit turbulence 90 miles from Kansas City, Mo.

The plane was diverted to Denver International Airport, where it landed safely around 7:45 p.m. and was met by medical crews.

Health officials said 21 people were treated at Denver hospitals and released by Wednesday. United said four flight attendants were among the injured. One other person was treated by paramedics at the airport.

Authorities initially said one person was critically injured, but Denver medical officials said Wednesday that appeared to be unfounded.

United spokesman Mike Trevino said some of the passengers were placed on another plane with a new crew and left Denver later Tuesday for Los Angeles.

The National Transportation Safety Board and the Federal Aviation Administration were investigating. The NTSB said Wednesday that investigators have received the information from the flight data recorder and will study that, the weather conditions and information from passengers and crew.

The Boeing 777 was taken to a hangar in Denver for inspection, and the FAA said no obvious damage to the exterior was found. There was minor damage inside.

Bechaz, who was heading home to Melbourne, Australia, via Los Angeles, said the flight had been smooth until a few minutes before the turbulence. The "fasten seat belt" sign went on and the flight attendants were seated, she said.

Then, "the whole plane felt like it was dropping," she said.

Bechaz said her seat belt was fastened and she stayed in her seat, but she was drenched in tomato juice, which also splashed onto the wall of the plane.

The National Weather Service said a line of strong thunderstorms extended from the middle of Missouri through the middle of Kansas Tuesday evening. With updrafts of up to 100 mph, thunderstorms can cause bumpy rides for airplanes as they pass from an area of calm air to churning air, much like a speedboat 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.

FAA spokesman Ian Gregor said the agency had issued a warning to the aircraft about thunderstorms before it hit the turbulence. It wasn't immediately known what action the pilot took.

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 and was in northern Oklahoma when it turned northwest toward Denver.

FAA spokesman Mike Fergus said he didn't know if any other planes were in the area or whether they had encountered turbulence.

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 plane was diverted to Montreal.

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Associated Press Writers Catherine Tsai and Colleen Slevin in Denver and Joan Lowy in Washington contributed to this report.