An empty passenger bridge collapsed Friday over the wing of a United Airlines plane at Denver International Airport, damaging the aircraft but causing no injuries, the airline said. The cause of the collapse and the extent of the damage were not immediately known. "The Boeing 757 had just arrived from Boston and the 184 passengers and crew were still on board," United spokeswoman Megan McCarthy said. "The collapse occurred in the rear portion of the bridge, the aircraft will be moved and repaired after the bridge is raised." McCarthy said.