In a split second and without warning, the plane began shaking violently in the sky.
Passengers panicked and screamed. One woman called out for her baby.
Another, they said, hit the ceiling so hard that she cracked the panel above her head.
United Airlines Flight 1676, carrying 114 passengers and five crew members, was headed from Denver to Billings, Montana.
Just as it was about to land, turbulence struck. The plane started convulsing.
"There was a lot of screaming, a lot of hollering," Bill Dahlin told CNN affiliate KTVQ.
Three crew members and two passengers were taken to local hospitals.
All were treated and released except one flight attendant, the airline said.
Dahlin said the turbulence apparently took the flight crew by surprise.
"I think they were trying to assess things themselves, so they really didn't offer any explanation because of what happened so quickly," he told KTVQ.
Another passenger, Laurel Linde, told CNN the sudden turbulence felt like a fall.
"It almost felt like an impact from below the plane. It was so sharp, and then things and people were flying everywhere," she said.
Some folks weren't strapped into their seats, like a man sitting just a row or two in front of Linde.
"He flew up out of his seat, hit the roof of the plane and ended up on the other side of the aisle on top of a woman on that side," she said.
United Airlines said its flight safety team will review what happened.
"Our primary focus is assisting our employees and passengers who were injured," United spokeswoman Christen David said.
Last month, a United flight returned to Newark Liberty International Airport in New Jersey after turbulence injured five flight attendants.
The Beijing-bound United Flight 89 hit "severe turbulence" about 45 minutes into the flight, officials said.
According to the FAA, turbulence can be unexpected and can happen even when the sky appears to be clear.