An Air Force colonel who said communications problems would cause "friendly fire" accidents will testify at a hearing into the accidental deaths of four Canadian soldiers in Afghanistan last year, a defense lawyer said Monday.

The hearing, which begins Tuesday, will generate a recommendation on whether two U.S. pilots should be court-martialed for dropping a 500-pound bomb on the Canadians near Kandahar on April 17, 2002.

Majs. Harry Schmidt and William Umbach of the Illinois National Guard are charged with involuntary manslaughter, aggravated assault and dereliction of duty. Air Force officials have declined to discuss the case.

David Beck, Umbach's defense lawyer, said the bombing occurred because the pilots were not told the Canadians would be practicing anti-tank maneuvers with live ammunition that night. Schmidt dropped the bomb after seeing anti-aircraft fire on the ground and believed Umbach was under attack.

Beck said Col. David Nichols, the pilots' commander, had warned his superiors in e-mails sent months before the incident that communications problems would result in accidental bombings of allied troops.

The hearing is similar to a civilian grand jury. Schmidt and Umbach face up to 64 years in military prison if the case goes to a court martial and they are convicted.

The decision on whether to court-martial the pilots will be made by Lt. Gen. Bruce Carlson, commander of the 8th Air Force based at Barksdale.