An unmanned U.S. spy plane crashed in Pakistan on Wednesday.

The U.S. Central Command, which runs American military operations in and around Afghanistan, said the pilotless Global Hawk crashed at 12:05 a.m. EDT while on a mission in support of the war against terrorism. It did not specify the location, but Pentagon officials said it was Pakistan.

Central Command said the aircraft was not downed by hostile fire. Pentagon officials, speaking on condition of anonymity, said the cause appeared to be engine failure.

It was the second Global Hawk aircraft to crash since the war in Afghanistan began last October. The first, in late December, went down in an undisclosed country near Afghanistan. The Air Force said last week that an accident investigation pinpointed structural failure as the cause of that crash. The investigators traced the problem to an improperly installed bolt that caused a control rod to fail.

The officials said U.S. troops had been dispatched to the crash site in Pakistan to examine the wreckage.

The Air Force has only about a half dozen Global Hawks. The exact number is secret.

The Global Hawk, made by Northrop Grumman and equipped with still-image cameras and sophisticated radar, is still in development. Eventually it is expected to replace the Air Force's U-2 spy plane, which has been operating worldwide since the 1950s.

The Pentagon made the unusual decision last fall to use the Global Hawk in connection with the war in Afghanistan even though it had not completed normal developmental tests. Among its uses: finding targets for U.S. strike aircraft.

The Global Hawk can fly continuously for up to 35 hours. With a wingspan of 116 feet, it is much larger than the Predator, an unmanned aerial vehicle also at work in Afghanistan. The Predator flies at lower altitudes and provides real-time video images of ground targets. An armed version of the Predator can fire Hellfire anti-armor missiles.

From takeoff to landing, the flight of a Global Hawk is controlled by an onboard computer that is programmed in advance. The Predator, by contrast, is controlled remotely by a "pilot" on the ground.