U.S. pilots who dropped a bomb on Canadian allies in Afghanistan last April quickly grew worried that they had made a mistake, according to audio and video recordings of the deadly accident played at a military hearing Wednesday.

Less than three minutes after the bomb hit, killing four Canadians and wounding eight, Illinois National Guard Maj. Harry Schmidt said: "I hope that was the right thing to do."

"Me too," said Maj. William Umbach, his mission commander.

The pilots maintain they believed enemy ground troops were firing at them, and defense lawyers later Wednesday said the video had been edited and was misleading.

The hearing is to determine whether the two pilots should face a court-martial on charges of involuntary manslaughter for the friendly-fire accident.

The Air Force has said the pilots failed to make sure there were no allied ground troops in the area. The pilots could get up to 64 years in prison if convicted.

Much of the conversation heard on the recordings had been transcribed and released last year, in the Air Force's unclassified report on its investigation of the April 17 bombing.

In the recording, taken from Schmidt's F-16, Schmidt spots flashes on the ground and requests permission to fire his cannon.

He is told by superiors to wait. Umbach says: "Let's just make sure that, uh, that it's not friendlies, that's all."

A short time later, Schmidt is heard saying: "OK, I've got ... some men on a road and it looks like a piece of artillery firing at us. I am rolling in, in self-defense." That meant he was rolling in to drop the bomb. The video shows a large explosion.

The flashes he had seen turned out to have been from Canadian soldiers who were conducting live-ammunition exercises near Kandahar.

Defense lawyers alleged the videotape had been doctored by deleting long stretches in which Schmidt and Umbach were relaying information to airborne air traffic controllers in an attempt to determine whether the ground troops were allies.

After the hearing, Schmidt's lawyer, Charles Gittins, said it was the Air Force's responsibility to provide pilots with information on whether fire on the ground comes from allies.

Earlier Wednesday, Col. Lawrence Stutzriem, who at the time was with the agency responsible for all coalition air operations, testified that Schmidt's cannon request "was extremely unusual."

Stutzriem said a pilot would normally give the command center time to investigate the source of the fire before asking permission to use the cannons, partly to allow time to learn whether there are allies on the ground.

"Telling apart friendlies from enemy (is) very difficult in Afghanistan," Stutzriem said.

Also Wednesday, four Canadian soldiers testified that the machine guns and anti-tank rockets their men were firing were not aimed into the sky when they were bombed.

Sgt. Lorne Ford, one of the wounded, said he was observing the group's live-ammunition exercise near Kandahar when he heard a jet overhead, and then blacked out.

"I woke up on my right side and I noticed my injuries right away," Ford said. His left leg had to be amputated, and he also lost his right eye.

He and three other Canadians -- Cpl. Brian Decaire, Cpl. Brett Perry and Master Cpl. Curtis Hollister -- testified that none of their men had aimed skyward while firing machine guns and rocket-propelled anti-tank weapons. Capt. Joseph Jasper said the same thing Tuesday.

The five men said they did not see any rounds ricochet more than 2,000 feet into the air.

Gittins said the pilots were wearing night vision goggles, which make all ground fire appear to be directed at the jet.

The proceeding is akin to a civilian grand jury hearing. A commanding officer will decide if the pilots will be court-martialed.