WASHINGTON -- Two Air Force F-16 pilots have been charged with involuntary manslaughter and assault for recklessly dropping a bomb in April on Canadian troops engaged in live fire training exercise near Kandahar in southern Afghanistan, the Air Force announced Friday.

The charges, which Air Force officials said were the first for involuntary manslaughter filed against Air Force pilots, came after a joint U.S.-Canadian investigative board found that one of the aviators dropped a laser-guided bomb less two minutes after he had been instructed to "hold fire" and seconds before controllers radioed to say allied forces were in the area.

The incident, which killed four Canadian soldiers and wounded eight others, triggered consternation in Canada, where a member of parliament criticized President Bush for failing to comment publicly in the immediate aftermath of the tragedy. Bush expressed his "heartfelt sympathy" a day later.

Defense officials identified the pilot who dropped the bomb as Maj. Harry Schmidt, a full-time instructor pilot at the Illinois Air National Guard's 183rd Fighter Wing. He has been charged with four counts of involuntary manslaughter and eight counts of assault as well as failing to exercise flight discipline and disregarding the Pentagon's rules of engagement governing the Afghan conflict.

Maj. William Umbach, a reserve pilot at the 183rd Fighter Wing who commanded the two-plane formation, has also been charged with involuntary manslaughter and assault as well as negligently failing to exercise appropriate flight command and failing to ensure compliance with the rules of engagement in not ordering Schmidt to hold fire.