Federal aviation investigators are looking into a United Airlines flight that encountered turbulence so severe that a flight attendant and a passenger were injured.

The Denver-to-Phoenix flight hit violent turbulence the night of April 15 about 30 minutes into the flight and the turned back to Denver.

United spokeswoman Robin Urbanski says the passenger was treated for a head injury and the flight attendant was treated for a sprained knee. Both were released that night.

National Transportation Safety Board spokesman Terry Williams says investigators decided to look closer at the cause of the turbulence after they examined the plane's flight recorder.