BILLINGS, Mont. -

Federal officials say a United Airlines flight from Denver to Billings likely encountered severe clear-air turbulence over Wyoming, causing a sudden loss of altitude that injured 11 of the 114 people on board.
    
The report from the National Transportation Safety Board said the Boeing 737-700 sustained minor damage in the Feb. 17 incident but continued for an uneventful landing.
    
Two people were seriously injured, including a flight attendant who remained hospitalized for several days.
    
Nine people reported minor injuries, including a lap-held infant who passengers said was wrenched from a parent's hold as the plane lurched violently while travelling at an altitude of 34,000 feet.
    
Clear weather was reported at the time of the event.
    
United has since returned the plane to service.