AP Military WriterWASHINGTON (AP) -- An American F-16 pilot in Afghanistan apparently mistook Canadian soldiers for enemy forces and thought he was acting in self-defense when he dropped a 500-pound bomb, killing four and wounding eight, U.S. officials said Thursday.

As an investigation began, a key question was why the Americans didn't know the Canadians were training in the area.

President Bush conveyed his 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 the first Canadians killed in a combat zone since the 1950-53 Korean War. Their unit is 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 immediately be confirmed that the pilot was a member of that unit. The pilot's name was not released, in accordance with usual U.S. military procedure in the course of an accident investigation.

The jet, flying in tandem with another American F-16, had been sent out on patrol. It was unclear whether they had been given any other mission.

It appeared the pilots did not know they were flying over an area restricted to training -- and so 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, he 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 that 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.

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 isn't complete, 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.