An American pilot who mistakenly bombed Canadian troops in Afghanistan last year, killing four of them, was under orders to hold fire when he dropped the bomb, a fellow airman testified Friday.

However, Maj. John Milton also said "hold fire" orders do not apply when pilots believe they are under attack.

Milton spoke at a hearing to determine whether two members of his Illinois National Guard squadron should be court-martialed. Majs. Harry Schmidt and William Umbach are charged with involuntary manslaughter and could face up to 64 years in a military prison if convicted.

An audio and videotape of the accident, taken from Schmidt's F-16, has been a key piece of evidence during the hearing and it was played again Friday.

On it, a flight controller is heard saying "hold fire" after Schmidt requests permission to fire his 20 millimeter cannons. Schmidt had spotted ground fire and thought Umbach was under attack.

Milton said the order meant Schmidt must refrain from attacking.

But just four seconds later, Schmidt said he was "rolling in" to drop the guided bomb and he did so 39 seconds after the "hold fire" order. Besides killing the four soldiers, the blast wounded eight other Canadians who had been performing anti-tank exercises with live ammunition.

Survivors testified earlier they were not firing into the air at the time. Defense lawyers have suggested the pilots thought they were under fire from enemy forces.

Under cross-examination, Milton indicated that a hold-fire order does not apply when a pilot believes he is under attack. Schmidt can also be heard on the recordings citing self defense at the same time he said he was "rolling in."

Milton, who has flown similar F-16 missions over Afghan combat zones, was not involved in the April 17 bombing. He was called as a government witness to explain, as a pilot, how he understood the events leading up to the bombing.

The defense has suggested that a breakdown in military communications kept the pilots from knowing there were allied troops in the area that night.

Milton indicated he was never briefed or given written materials about allies at Tarnak Farm, the firing range near Kandahar where the Canadians held their anti-tank exercises the night of the bombing.

"There is no doubt in my mind that (the bombing) never would have happened" had the pilots been informed the Canadians would be in the area, he said.

Milton's statement contradicted earlier testimony from Col. Lawrence Stutzriem, who at the time of the bombing was with the agency responsible for coalition air operations. Stutzriem said Air Force pilots flying missions in the area had received written orders warning that allied troops would intermittently use live ammunition.

Milton said the 40-page booklet was "unworkable" because it included so much information. The pilot also said an investigation into communication problems would do more than the hearing to prevent friendly fire incidents.

"This should be a safety investigation, to find the problem and fix the problem," Milton said.

Prosecutors also asked Milton about amphetamines issued to pilots by the Air Force to prevent fatigue. Defense attorneys have suggested the "go pills" could have affected the pilots' judgment, a contention disputed by the Air Force.

Milton said he talked just once with Schmidt about Air Force-issued amphetamines: He said they discussed a report about "go pills" aired on the ABC News program "20/20" in December, eight months after the bombing.

Later Friday, Air Force helicopter pilot Maj. Trevor O'Day testified that he saw some small arms fire while flying at about 100 feet in the Tarnak Farm area on the night of the bombing. O'Day said he did not feel threatened and saw no shots being fired skyward, but decided to steer clear of the area.

The hearing was to resume Saturday.

Schmidt is a combat-decorated Navy pilot who transferred to the National Guard in 2000. Umbach is a United Airlines pilot who had served in the Air Force. They are also charged with aggravated assault and dereliction of duty.

After the hearing, expected to last two weeks, a commanding officer will decide if the pilots will be court-martialed.

Killed in the blast were Sgt. Marc Leger, Cpl. Ainsworth Dyer, Pvt. Richard Green and Pvt. Nathan Smith, all members of the 3rd Battalion of the Princess Patricia's Canadian Light Infantry, which is based near Edmonton. The deaths were the first in a Canadian military combat operation since the Korean War.