On a clear night last April, former Top Gun flight school instructor Harry Schmidt was on a mission over Kandahar in Afghanistan. His commander, Major William Umbach, radioed: 'I've got some men on a road and it looks like a piece of artillery firing at us . . . I am rolling in, in self-defence.' But it was Schmidt who fired, unleashing a single laser-guided 227kg bomb at the target, which proved to be Canadian troops on a live-fire exercise. Four Canadians died - Sergeant Marc Leger, Corporal Ainsworth Dyer, Private Richard Green and Private Nathan Smith - and eight others were wounded. In the wake of fierce criticism from Canada and a joint US-Canadian inquiry which blamed the pilots, Major Umbach accepted a reprimand and retired from the Air Force. But in September 2002 Schmidt, who is nicknamed 'Psycho'' by his F-16 comrades and denies blame, was charged with four counts of negligent manslaughter, eight counts of aggravated assault, and one count of dereliction of duty. If convicted, he could face 64 years in prison. As the case is set to start, debate rages about the true cause of the incident. Schmidt claims that 'go pills' played a significant part. Clinically known as dextroamphetamine and commercially sold as Dexedrine, the pills are essentially 'speed'. Schmidt claims he was routinely pressured into taking the stimulant as part of an American Air Force drive to produce 24-hour warriors. The two he had taken on the night of the incident supposedly impaired his judgment. The company that manufactures the pills, GlaxoSmithKline, refused to comment. However, an Air Force spokesman, Captain Wes Ticer, conceded that as with any drug, Dexedrine has listed possible side effects. According to Drugscope, the British-based drug-information service, symptoms can range from panic attacks and paranoia to violent mood swings and high aggression. But Captain Ticer underlined that, before taking any go pills, each pilot is screened to determine if he or she experiences any side effects. He denied that pilots are forced to take the pills - pilots have a choice but are only meant to use them 'as a last resort' on long-duration missions of over 20 hours. The pills are not intended to be used as a routine replacement for sleep. They are nonetheless useful, Captain Ticer argued, saying the problem they are meant to combat - fatigue - is itself 'very dangerous'. Military analyst Jess Johnson, a former US combat medic and special forces weapons instructor, also cast doubt on whether go pills should be viewed with suspicion. Mr Johnson said they make an 'effective' defence in court 'but there are so many variables to explain these deaths, it is not a strong defence'. Variables include the pilots' claim that they were simply not briefed about the Canadian army exercises. The Air Force denies this. Either way, it seems unreasonable to entirely dismiss the amphetamines issue, which Schmidt's lawyer Charles Gittins insists will be a major factor in the defence strategy. Mr Johnson himself admits that 'these compounds change the perceptions and judgment of the individual'. Evidence that Schmidt's perceptions were profoundly askew during the incident comes from Colonel Lawrence Stutzriem, deputy director of Air Force air operations in Afghanistan, who was on duty the night the gun battle happened. Colonel Stutzriem has described the level of aggression Schmidt displayed before the attack as 'extremely unusual', referring to a request Schmidt made - which was rejected - to strafe the target before he dropped the bomb. The go pills defence is further corroborated by John Pike, director of the defence think-tank Globalsecurity.org. According to Mr Pike, Schmidt behaved in an 'inexplicably aggressive' manner beyond weariness, inexperience or recklessness. 'The simplest explanation is that the guy had eaten too much speed and was paranoid,' Mr Pike is on record as saying. The amphetamine defence will doubtless take some of the heat off Schmidt, who is said to be in turmoil about his predicament, obsessively replaying the incident in his mind. 'Every day he has to try to handle the pressure, the criminal charges, the future, the accusations, the feeling of knowing that the bomb killed men who were allies. Hard is not even a word for it,' his wife said. But the pressure to blame a person instead of a pill is intense. Mr Lloyd Smith, the father of victim Nathan Smith, is quoted as saying: 'I'm a professional person myself; I'm a captain on an oil tanker. And I understand what responsibility and that is all about, and with that comes accountability for the actions and the decisions you make.' The pressure on the Air Force to punish Schmidt despite his Top Gun status is all the more intense because the deaths still resonate with haunting symbolic significance. They are remembered in Canada as the first the peace-loving country had suffered since the Korean War.