Among questions facing investigators of the latest bombing accident in Afghanistan, in which four Canadian soldiers were killed and eight wounded, is why the American pilot who dropped the bomb apparently was unaware Canadian soldiers were training in the area.

Also, why didn't the air controller who communicated with the pilot tell him the Canadians were there?

The pilot, on a nighttime air patrol in an F-16 jet fighter, apparently mistook the Canadians for enemy forces and thought he was acting in self-defense when he dropped a 500-pound bomb Thursday, U.S. officials said.

Canadian officials demand an explanation.

President Bush conveyed the nation's regret to Prime Minister Jean Chretien, and Defense Secretary Donald H. Rumsfeld said U.S. and Canadian officers at Central Command headquarters in Tampa, Fla., would work together to piece together what happened and why.

The soldiers were from the 3rd Battalion of the Princess Patricia's Canadian Light Infantry, based near Edmonton, Alberta.

In a brief announcement, Central Command said an Air National Guard F-16 aircraft dropped one or two laser-guided bombs on the Canadians, but it offered no other details. Pentagon officials said the Canadians were conducting a nighttime, live-fire training exercise near the southern Afghan city of Kandahar.

The F-16 was from the 183rd Fighter Wing, whose home base is Springfield, Ill., but it could not be confirmed immediately whether the pilot was a member of that unit. The pilot's name was not released, in accordance with usual U.S. military procedure during an accident investigation.

The jet was flying in tandem with another U.S. F-16.

It appeared the pilots did not know they were flying over an area restricted to training, and the fire from the training exercise made them believe they were under attack, officials at the Pentagon said. All forces operating in the Kandahar area are supposed to be aware of friendly forces.

One of the pilots sought permission to bomb and was told to mark the target but not fire, a senior Pentagon official said. On a second fly-around, after reporting he was taking ground fire, the official said the pilot dropped the bomb in what he thought was self-defense.

Canadian Defense Minister Art Eggleton, who called the deaths shocking, said one of the injured had life-threatening wounds and the other seven were in stable condition.

Chretien addressed the national Parliament in Ottawa.

"We have so many questions this morning," he said. "Extensive training for combat is meant to save lives. How does this happen? In this awful case it took so many lives, and I want to assure the families and the people of Canada that these questions will be answered."

A Canadian representative at U.S. Commander Tommy Franks' headquarters in Tampa, Capt. Isabelle Compagnon, said the joint U.S.-Canadian investigation would determine whether the Canadians had followed normal procedures in notifying other coalition forces, including the Americans, of the time and place of their training exercise.

Canada's defense chief, Lt. Gen. Ray Henault, told reporters in Ottawa the area was recognized as a training sector and the aircraft were using strictly controlled routes.

"How this can happen is a mystery to us. Without a doubt, there was a misidentification," Henault said.

Rumsfeld refused to discuss details of the apparent accident during the investigation. "What we do know is that some very fine coalition partners of ours ... were killed by one or more bombs ... dropped by one or more F-16s," he said during a visit to Scott Air Force Base, Ill.

The bombing was among the worst friendly fire accidents since the U.S.-led campaign in Afghanistan began in October.

On Dec. 5, a B-52 dropped a bomb on U.S. and Afghan forces near Kandahar, killing three Americans and at least seven Afghans, and slightly wounding Hamid Karzai, now Afghanistan's interim leader. The investigation is incomplete, but officials have said there were errors in transmitting target coordinates to the B-52.

On Dec. 22, U.S. aircraft struck a convoy near Khost, killing dozens of Afghans. Some Afghans say the convoy was carrying tribal leaders to Karzai's inauguration, but U.S. military commanders insist it was a legitimate target.