(CNN) -- U.S. transportation safety investigators will launch a formal probe of an incident this week involving a United Airlines jet that was rocked by severe turbulence, injuring five people.
The National Transportation Safety Board said it would investigate Flight 1676, which experienced a severe midair upset just before landing on Monday in Billings, Montana, on a flight from Denver.
The captain declared a medical emergency due to the injuries and the plane landed safely.
Passengers described a harrowing scene as the Boeing 737 carrying 114 passengers and five crew members suddenly shook violently during descent.
Passengers panicked and screamed, one of those aboard said.
United is also reviewing the incident.
Turbulence can be unexpected and can happen even when the sky appears to be clear, the Federal Aviation Administration said.