The Tyrone man in the driving seat on the day JFK was gunned down in Dallas
He was an eyewitness to history, a farmer's son from rural Tyrone who lived the American dream and became a key figure in one of the defining moments of the 20th century.
William Greer was sitting just feet from John Kennedy when an assassin's bullets struck down the President half a century ago on November 22, 1963.
He was at the wheel of the presidential limo which was snaking its way through Dealey Plaza in Dallas, Texas, when Kennedy was fatally wounded.
A recent best-selling book also pointed the finger of suspicion at Greer's role in the assassination, amid claims he didn't react quickly enough or – as some conspiracy theorists claim – actually fired the fatal shots.
Born in 1909, Greer's early years were spent on the family farm at Stewartstown. He emigrated to the United States in February 1930.
After arriving in America he worked in Boston, and later moved to New York where he was employed as a chauffeur.
He served in the US Navy for three years before joining the Secret Service in October 1945.
Five years later, Greer joined the White House staff and, over the next 13 years, worked as a chauffeur for Presidents Harry Truman, Dwight Eisenhower and Kennedy.
But his Kennedy mission ended in tragedy during a presidential visit to Dallas on November 22 1963.
Greer described events in a testimony to the Warren Commission, an inquiry set up to investigate Kennedy's assassination, in 1964.
Also in the car were the First Lady Jacqueline Kennedy, Governor of Texas John Connally and senior Secret Service agent Roy Kellerman.
Greer told the inquiry: "When we were going down Elm Street, I heard a noise that I thought was a backfire of one of the motorcycle policemen.
"We had had so many motorcycles around us. So I heard this noise. And I thought that is what it was.
"And then I heard it again. And I glanced over my shoulder. And I saw Governor Connally like he was starting to fall. Then I realised there was something wrong.
"I tramped on the accelerator, and at the same time Mr Kellerman said to me, 'Get out of here fast'. I cannot quite remember any more."
Greer recalled hearing three shots.
Greer helped rush Kennedy to nearby Parkland Hospital, where he was pronounced dead at 1pm local time.
Amid a wealth of conspiracy theories over the past half century are some focusing on Greer's role on the day.
The Kennedy Detail, a recent book written by former Secret Service agent Gerald Blaine and journalist Lisa McCubbin – soon to be the basis of a film – repeated allegations that if Greer had acted faster when the shots were fired the President may have lived.
Senator Ralph Yarborough was riding with Vice-President Lyndon Johnson, and was very critical of Greer's response.
"When the noise of the shot was heard, the motorcade slowed to what seemed to me a complete stop," he said.
"After the third shot was fired, but only after the third shot was fired, the cavalcade speeded up, gained speed rapidly, and roared away to the Parkland Hospital."
Whatever the truth about his role that day, we are unlikely to discover it. William Greer died in February 1985 aged 75 from cancer. He is buried in Green Hill Cemetery in Waynesville, North Carolina.
Conspiracy theorists are still having a field day, 50 years on
Lee Harvey Oswald may have fired the shots, but doubts over whether he acted alone have intrigued conspiracy theorists for years.
US Secretary of State John Kerry recently added to the intrigue when he claimed he had "serious doubts" that Oswald acted alone. One theory is that Cuban leader Fidel Castro ordered Kennedy to be murdered because the President tried to have him killed.
A second version states that Soviet agents – again, using Oswald as a foil – killed Kennedy because the President had embarrassed Premier Nikita Khrushchev in the missile crisis showdown.
Lyndon Johnson is a key figure in any number of theories. Motives range from the simple – he wanted to be President – to the far-fetched. Some say he was working with a cabal of rich tycoons who wanted to end Kennedy's liberal social policies. Another suspect, the Mafia, liked Kennedy's religion but hated his politics.
There are many theories about CIA involvement, including a rogue cell or a CIA contract killer gone bad.