To John O’Neill, it all came down to Osama bin Laden. The FBI counterterrorism expert studied the Saudi fugitive’s every move beginning in the wake of the 1993 World Trade Center bombing. O’Neill was quick to suspect bin Laden’s involvement in many of the attacks leading up to that of Sept. 11, 2001. He told his colleagues and friends that the terrorist leader was plotting something big — and that the Twin Towers were likely to be hit again.

Ironically, O’Neill — who had left the FBI in August 2001 to become the World Trade Center’s security chief — was killed in the South Tower on Sept. 11, another al Qaeda victim.

What if his superiors in government had paid attention to his warnings? Could 9-11 have been avoided? In his biography of O’Neill, The Man Who Warned America, New York Post criminal justice editor Murray Weiss suggests that the outcome could have been different.

“It is impossible to know with any certainty what might have happened if O’Neill had still been on the job and fully engaged in the fight against bin Laden, instead of heading out the door,” Weiss writes, adding that if various “miscues” in the period leading up to 9-11 “had been handled in ‘O’Neill fashion,’ they might have led to the exposure of bin Laden’s plan to attack the World Trade Center before it was too late.”

Weiss, who met O’Neill in the course of his reporting work and got to know him over the years, is clearly a fan of the controversial FBI agent. A flamboyant character who favored high-style black suits; wore a pinkie ring; and dated many women simultaneously; O’Neill had his admirers and his detractors within the bureau. He worked long hours, retaining vast amounts of information about the cases under his jurisdiction, and then — apparently needing almost no sleep — would go out on the town until the early morning.

Born in Atlantic City in 1952, O’Neill, the only child of taxi-driving parents, went to Catholic school; worked as a lifeguard; and yearned to become an FBI agent. A focused young man, he enrolled at American University; married high school sweetheart Christine; clerked at the FBI and became the father of a son — all by age 20.

Selected as an agent in 1976, he climbed from one success to another: learning the ropes in Baltimore; combating gangs in Chicago; and finally taking on the terrorist world from Washington and New York. He investigated everything from the Khobar Towers bombing in Saudi Arabia to the U.S.S. Cole incident in Yemen, embracing each assignment with gusto.

But his workaholic ways, combined with his philandering, caused his marriage to suffer. After having two children, O’Neill and Christine decided to live apart — but never divorced. O’Neill was involved with numerous paramours, including three long-term girlfriends he was still seeing at the time of his death: Valerie James (whom he met in Chicago and lived with in New York), Anna DiBattista and Mary Lynn Stevens.

O’Neill — also the subject of a PBS “Frontline” documentary similarly titled “The Man Who Knew” — lied to them all, Weiss writes. He hinted at or suggested marriage to each of them; portrayed himself as divorced; and repeated that there were no other women in his life. The women only learned of one another’s existences after O’Neill’s death.

In the end, O’Neill and the FBI parted ways after he broke the rules a couple of times — once allowing James into a restricted FBI facility to use the ladies’ room, another time leaving his classified briefcase unattended for several minutes at a meeting — and fell afoul of the bureau hierarchy. Wishing to stay in New York, a city he adored, he opted for the Trade Center job.

While Weiss depicts O’Neill’s actions in a primarily favorable light, even the multiple-girlfriend issue, where Weiss seems bemused rather than appalled, he lashes out at the Clinton administration, portraying it as so preoccupied with the Lewinsky scandal and domestic politics that it failed to pay enough attention to the danger posed by international terrorists.

The book gallops along in a lively fashion from one O’Neill adventure to another, dealing with some extremely serious issues along the way. It is marred only by sloppy editing, including many typos. But if you want to learn more about a fascinating figure and his fight against terrorism, this book is well worth reading.