Two American pilots accused in a fatal "friendly fire" incident in Afghanistan took amphetamines before it occurred, an enquiry has found.
The pilots were taking the drugs prescribed by the United States military when they killed four Canadians last April, according to US Air Force investigators.
They are awaiting a court martial hearing due to start next month.
US Air Force majors Harry Schmidt and William Umbach face up to 64 years in prison following the incident over Kandahar.
The pilots thought they were being fired at and dropped a laser-guided missile on their presumed attackers.
It was only when they had killed four people and injured eight that they were told the group were Canadian soldiers taking part in a night live-fire exercise and that they had informed the US military.
But it has now emerged that about an hour before the incident the two men had taken amphetamines, or speed as it is commonly known, that had been given to them by the US Air Force.
The pills, which are illegal in the US, are given to combat pilots who are involved in long eight or nine-hour sorties in small controlled doses, say the military.
The Air Force stopped prescribing the 'Go' pills, as they are known by the pilots, in 1993 after reports that crews using them during the Gulf War became addicted.
But the drug has been quietly reintroduced in recent years.
The Air Force says they are a medical tool that is essential for combat pilots being sent to war over Afghanistan and, possibly, Iraq.