Branded indifferent by some Canadians, President Bush emphasized Friday his regret for the Canadian soldiers killed and injured in a U.S. bombing accident, saying "I wish we could bring them back."

As investigators began to question why the American pilot who dropped the bomb did not know Canadians were training in that area of Afghanistan, Bush pledged: "We'll find out what happened. It was a terrible accident."

Another question for investigators is 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.

Bush, who telephoned Prime Minister Jean Chretien immediately after the accident Wednesday night, touched a nerve in the Canadian press when he made no public comment about the accident all day Thursday except for a written statement released late in the afternoon.

Friday morning editions of the Toronto Globe and Mail carried a headline that read, "Deaths don't merit a mention from Bush; U.S. indifference."

While touring a Secret Service training facility, Bush appeared eager to remedy the Canadian impression. He approached reporters and, without prompting, offered this statement:

"I want to say publicly what I told Jean Chretien the other day — about how sorry I am that Canadian soldiers lost their lives in Afghanistan. It was a terrible accident. Parents and loved ones of the soldiers have my most heartfelt sympathy and I wish we could bring them back. But we can't. I appreciate so very much the sacrifices that the Canadians are making in the war against terror and again I'm so sorry this accident took place."

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.

Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said Thursday that he could not explain what happened.

"It sounds, you know, just almost inconceivable that certain things could happen this way," Myers said. " ... It's right up there with the worst news I've heard in my career."

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.