NEW YORK (AP) — John P. O'Neill was something of a new man when he strode through The Plaza Hotel's grand doors on Sept. 8.

He had come to toast friends on their wedding day, but he had his own reasons to celebrate: He had recently landed the World Trade Center's top security job after leaving the FBI amid concerns over his aggressive style and the temporary loss of a briefcase containing secret documents.

It was a plum private-sector reward for a public-service career in counterterrorism. He didn't live long enough to savor it.

O'Neill died in the crumbling inferno authorities say was most likely Osama bin Laden's most horrific attack. The kamikaze hijackings that killed O'Neill and perhaps nearly 7,000 others across the East Coast were just the type of large-scale terrorist act he foretold.

O'Neill spent his last years almost single-mindedly stalking bin Laden and warning that a major terrorist attack on American soil was a matter of when, not if.

"Almost all of the (terrorist) groups today, if they choose, have the ability to strike us here in the United States," O'Neill said in a 1997 speech. "They're working toward that infrastructure."

Just three days before the attacks, O'Neill told former New York City police commissioner Howard Safir that he planned a security review to look for chinks in trade center security. The two dined together at the black-tie wedding reception.

"He was excited about a new opportunity. We always used to joke all the time that there was life after government," Safir said.

They also talked about "smart doors" that recognize faces and fingerprints to determine instantly whether someone should be allowed into a building or room.

"He would have been looking at new technology constantly," Safir said. "John always believed that we were a high-priority target and worried about it all the time, as we all did. And John was someone to take seriously when he told you he was worried."

Both Safir and Frank Cilluffo, a counterterrorism specialist with the Center for Strategic and International Studies in Washington, said O'Neill's experience investigating the 1993 bombing at the trade center — linked by authorities to bin Laden — made him uniquely qualified to head security there.

It was that attack, which killed six and injured more than 1,000, which sharpened O'Neill's appetite to combat terrorism.

"The ironic thing about John is that he has been chasing, to a certain extent, Osama bin Laden, and that when he retired he actually became a victim of Muslim extremists," said Barry Mawn, the FBI's assistant director in charge of the New York office.

O'Neill joined the FBI as a clerk shortly after graduating from high school in Atlantic City, N.J. Later, as a special agent, he worked in Baltimore, Washington and Chicago on counterintelligence, organized crime, public corruption, racketeering and fraud cases before facing terrorism head-on as an investigator in the first World Trade Center attack.

By 1995, he was counterterrorism chief at Washington headquarters, in charge of coordinating information about suspects in the Oklahoma City bombing.

Friends say O'Neill found his true niche when he moved to New York in 1997 as special agent in charge of the National Security Division. Already popular with law enforcement in Chicago — where authorities said he helped create unprecedented local and federal cooperation — the 6-foot-2, meticulously dressed O'Neill soon seemed to know everyone important in Manhattan.

He was a key investigator into the 1998 bombings of U.S. embassies in Kenya and Tanzania, and into last year's bin Laden-linked suicide attack on the USS Cole in Yemen.

But his sterling reputation collected some tarnish, too. Last year, during an FBI conference in a Tampa, Fla., hotel, O'Neill left the room to answer a page, only to return and find his colleagues gone to lunch and his briefcase — containing secret documents — gone.

Apparently stolen by a petty thief who mistook it for a laptop, the bag was quickly recovered. The infraction, however, was serious enough to prompt a Justice Department investigation and FBI probe.

O'Neill also ran into intergovernmental politics. His hard-charging style had offended some people in Yemen, and Ambassador Barbara Bodine blocked his return there.

Friends and colleagues say the high-paying trade center security job looked attractive against such a backdrop, though they insist O'Neill was under no pressure to leave the FBI.

"This was a very good opportunity," Mawn said. "He hated leaving, but you do get to a point where you have to look out for your family and for yourself."

Christopher Isham, who runs the investigative unit of ABC News and spent a weekend with O'Neill in the Hamptons in late August, said O'Neill had become frustrated by investigative hurdles, including privacy laws preventing FBI access to e-mail accounts terrorists used to communicate.

When the first plane struck the trade center Sept. 11, O'Neill is believed to have been in his 34th-floor office. He phoned a son and a friend to say he was OK, then returned to one of the buildings to help. His body was found last week.

Safir said O'Neill almost surely would have recognized the terrorist attack before the second plane hit 18 minutes later.

"He understood that terrorists are very patient, very calculating, very compartmentalized," Safir said. "They are a formidable adversary."