COLIN McLaren got the bad news the week he started at the police academy: his much-loved Uncle Neil had been killed in a shooting accident.

Neil McLaren was a sensible and seasoned shooter but he'd made a mortal error, getting into a car after a shooting trip without checking his shotgun. The car hit a bump, the gun went off and he was dead.

His nephew always had that loss hanging over him. He would spend years living dangerously in heavy squads and undercover jobs, dealing with the Walsh St murders and infiltrating Australia's Calabrian mafia. He carried guns, but carefully. He knew mistakes could be fatal.

During his time in the force, a policeman called Neil Clinch was shot dead by a policewoman aiming at an "offender" - who was, in fact, a householder fearing the police in his backyard were intruders.

Then there was Constable Clare Bourke, shot dead at Sunshine police station by a policeman fooling around with an "empty" pistol.

Meanwhile, plenty of other incidents went unreported, such as the one in which a future Assistant Commissioner and another cop were chasing a suspect in Windsor. One of them accidentally shot a passing taxi. The bad guy escaped; the innocent cab driver surrendered immediately.

Anyone who has handled guns knows mistakes happen - and that we don't always hear about it. McLaren was reminded of that in 1992 when he took a trip to New York to recover from a tough year investigating the "Mr Cruel" child abductions.

He picked up a book in Times Square for the flight to Chicago. The book, Mortal Error, outlined how a ballistics expert called Howard Donahue had proved beyond reasonable doubt that John F. Kennedy was hit in the head by a hollow-point bullet, not the conventional military rounds fired seconds earlier by Lee Harvey Oswald.

Donahue identified the origin of the fatal hollow-point - a Secret Service agent with an assault rifle in the open-top escort car behind the President's. The iconic Zapruder film of the Dallas motorcade shows the alarmed Secret Service man clutching the weapon as he tries to stand just after Oswald's shots strike from above.

The force of a simple story that fitted the facts satisfied McLaren's detective instincts. To an expert, the bullet fragments revealed a tragic accident caused by Oswald's crazy assassination attempt. This was no convoluted conspiracy theory, of which there were many, including the one peddled by Oliver Stone in his fictionalised hit film JFK. It seemed common sense.

But a dry, factual ballistics analysis was never going to compete for public attention with Kevin Costner starring in Stone's fictionalised entertainment.

McLaren realised the case needed an independent investigation to test if the other evidence supported Donahue's conclusion that Secret Service agent George Hickey had accidentally finished what Oswald had begun. Who better to do it than an outsider: an Australian investigator with no axe to grind?

McLaren was keen but first he had to see out his police career and finish other projects. He wrote two successful books based on his undercover work and built his hospitality business from scratch in northeastern Victoria.

Nearly five years ago, he started on the JFK project. As a detective, he says, he was happy to go "where the evidence takes us". He bought a 26-volume set of the official Warren Commission report. Then the 5000-page Assassination Records Review Board finding of 1993, which lifted secrecy provisions on material from 28 Government agencies.

It seemed clear that key players had strived to save the Secret Service huge embarrassment by hiding the fact that Kennedy's brain (which vanished immediately after autopsy) had been pulped by one of their own bullets.

media_camera Book cover - JFK The Smoking Gun , Colin McLaren

McLaren traced 22 witnesses who saw Kennedy shot. Ten had smelled gun smoke and 12 of them saw it at ground level near the Secret Service car. Hickey normally drove but had been handed the weapon because the security detail was shorthanded.

Witnesses revealed they had been intimidated and gagged before and during the Warren Commission hearings in 1964. But now they could tell all.

McLaren worked with a Canadian production house to film a documentary, which will be aired in North America and Australia (on SBS) next month to follow this week's launch of his book JFK: The Smoking Gun. Hickey died in 2011, which makes it easier to tell the story without the fear of a lawsuit. Hickey had attempted to sue Mortal Error's publishers but failed.

McLaren knows that the fact most Americans don't believe the official account of the assassination does not guarantee they will "buy" what he calls his "brief of evidence". But when he launches a three-week publicity tour of the US and Canada today (including the David Letterman show and a Wall Street Journal interview) at least he gets the chance to argue it was a fumbling accident, not a murky assassination conspiracy.

Diehard conspiracy theorists might consider the fact that in September 2006, a Secret Service agent accidentally fired his shotgun while guarding the visiting Iranian President. It would the Secret Service years to acknowledge that embarrassing fact, now the subject of a book.

Then there's the scandal of the death of an all-American hero, the former NFL footballer Pat Tillman who became a patriotic poster boy for the Afghanistan campaign when he quit a $3 million football contract to join the army after the 9-11 attacks.

Tillman's enlistment was such a public relations coup that when he was killed by a trigger-happy American soldier in 2004, the cover-up ran from his own commanders to the White House. Tillman's family were lied to for months about who killed their son. Which would make perfect sense to the Secret Service bosses who apparently covered up George Hickey's blunder for 50 years.

One thing is certain: guns go off in the darnedest ways and places. When Lee Harvey Oswald was a marine, he once accidentally shot himself in the leg with his service pistol.