Barksdale Air Force Base, La. ? Two U.S. pilots who mistakenly dropped a bomb that killed four Canadians in Afghanistan had been issued amphetamines before the mission to stay awake, a defense lawyer argued Tuesday at the opening of a military hearing to determine whether they should be court-martialed.

The Air Force-issued “go pills” may have impaired the pilots’ judgment, said David Beck, lawyer for Maj. William Umbach. He also said the pilots were given antidepressants following missions.

Umbach and Maj. Harry Schmidt are charged with involuntary manslaughter for dropping the guided bomb near Kandahar, Afghanistan, on April 17. The Air Force has said they failed to make sure there were no allied troops in the area.

Beck and Charles Gittins, Schmidt’s lawyer, have said the fighter pilots were not told Canadian troops were conducting live-fire exercises and believed their F-16s were under attack.

Beck said Tuesday that the Air Force issued amphetamines to help pilots stay awake during long missions. He promised to raise the issue later in the hearing. “The Air Force has a problem. They have administered ‘go pills’ to soldiers that the manufacturers have stated affect performance and judgment,” Beck said.

The Air Force has said that use of the pills is voluntary, and that their effects have been thoroughly tested.

The proceeding is akin to a grand jury hearing in the civilian justice system. Lt. Gen. Bruce Carlson, commander of the 8th Air Force based at Barksdale, will decide whether the pilots will be court-martialed for the friendly fire accident.

The two Illinois National Guard pilots also face charges of aggravated assault and dereliction of duty and could get up to 64 years in military prison if convicted.

The first witness called Tuesday was Canadian infantry Capt. Joseph Jasper, who said he heard a fighter jet as he prepared to direct a tank-stalking exercise. Then the bomb hit nearby.

Jasper said he did not recall any of the soldiers firing in the air, and he said the Canadians had not fired their weapons for several minutes when he heard the bomb blast.

“Basically we looked at each other and said, ‘What the hell was that?”‘ Jasper said.

The bodies of the four soldiers were soon found, and medics treated eight wounded soldiers, he said.