A US pilot who mistakenly bombed Canadian soldiers in Afghanistan says he thought he was acting in self-defence.
The Pentagon says the National Guard pilot was unaware he was flying over an area restricted to training.
Fire from the troops' training exercise made them believe they were under attack, US officials added.
A senior Pentagon official says one of the pilots sought permission to bomb and was told to mark the target but not fire.
On a second fly-around, after reporting he was taking ground fire, he dropped the bomb in what he thought was self-defence.
The 500lbs bomb killed four Canadian soldiers and wounded eight. They had been conducting a night-time, live-fire exercise near the southern Afghan city of Kandahar.
The troops were the first Canadians killed in combat since the Vietnam War.
The US jet, flying in tandem with another American F-16, had been sent out on patrol. It is unclear whether they had been given any other mission.
An investigation has been launched and will focus on why the Americans were unaware the troops were training in the area.
The pilot's name has not been released, as is usual US military procedure for accident investigations