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Sources: 'Friendly fire' airstrike likely human error

WASHINGTON (CNN) -- Human error, not mechanical malfunction, appears to have caused a "friendly fire" accident that killed three U.S. soldiers and injured 20 other special operations troops in Afghanistan in December, Pentagon sources said Tuesday.

An investigation into the December 5 incident so far has yielded only preliminary findings, a senior Pentagon official stressed. The final report on the accident is not due to be completed until the end of the month.

But sources says it appears the accident north of Kandahar was similar to one nine days earlier when five U.S. special forces soldiers were injured while calling in airstrikes to put down a prison uprising outside Mazar-e Sharif.

RESOURCES • Defense Department studied friendly fire problem after Gulf War  • U.S. weapons

In both cases, sources said, precision-guided, 2,000-pound bombs were mistakenly programmed with the location of U.S. troops on the ground instead of the intended target nearby. Confused communications between soldiers on the ground and a B-52 crew in the air prompted the accident, meaning U.S. commandos essentially called the airstrikes in on their own position, sources said.

The accident also killed five Afghan fighters and injured 18 other Afghans, including slight injuries to Hamid Karzai. He now heads the new interim Afghan government. The accident prompted military officials to tighten their procedures, Joint Chiefs Chairman Gen. Richard Myers said.

"There are ways to check and double-check your work. And there were … improvements made to help preclude it, as the investigation continues, because it's prudent stuff to do." Myers said.

The dead American soldiers were identified as Master Sgt. Jefferson Donald Davis, 39, of Watauga, Tennessee; Sgt. 1st Class Daniel Henry Petithory, 32, of Cheshire, Massachusetts; and Staff Sgt. Brian Cody Prosser, 28, of Frazier Park, California. All were in the 3rd Battalion, 5th Special Forces Group, based at Fort Campbell, Kentucky.