He was a flight surgeon with the Navy and a CIA officer in Pakistan. He has also earned a doctorate in sociology and written two books.

Now Marc Sageman can add a new entry to his resume: terrorism guru for the New York City Police Department.

Dr. Sageman, billed by the NYPD as its first-ever “scholar in residence,” has become a key player in a debate over whether the greatest terror threat America faces comes from inside or outside its borders.

His assignment: to teach terrorism workshops to investigators and be a sounding board for a team of NYPD analysts formed after the attacks of September 11, 2001 to assess future threats against the city.

“I think I bring a new voice, a new way of looking at things,” Dr. Sageman, 55, said in a recent interview.

Dr. Sageman’s residency at the nation’s largest police department began in May and will last a year, with a private foundation paying his $180,000 salary.

Dr. Sageman’s way of looking at the terror threat emphasizes local, loose-knit cells of alienated young men — what he calls “bunches of guys” — over established international networks like Al Qaeda.

In the post-September 11 terror plots that have emerged in Britain, Spain, the United States, and elsewhere, “It was just a bunch of guys getting together and then getting radicalized over time locally,” he said. “It wasn’t an outside organization coming in and brainwashing them. … People get radicalized at home and go out and look for Al Qaeda, not the other way around.”

Though not as sophisticated as the September 11 brand of terrorist, the wannabes pose a grave risk, he said.

“A lot of those guys are not the brightest guys around,” he said. “The vast majority get arrested. But some of those guys get lucky. And they can kill lots of people.”

Mr. Sageman was born in Poland, but spent most of his early youth in France. His family moved to Brooklyn when he was a teenager.

After earning degrees from Harvard and New York University and his stint in the Navy, he joined the CIA in 1984. He had postings in Islamabad and New Delhi until 1991, when he became a clinical and forensic psychiatrist.

Before the attacks of September 11, he said, he studied the motivations of hundreds of murderers. After, he looked at the mindsets of terrorists and warning of an emerging homegrown threat in his books, speeches at universities, and at law enforcement agencies across the country and abroad.

Mr. Sagemen caught the attention of the NYPD’s deputy commissioner of intelligence, David Cohen, also a CIA alum, and other NYPD brass when he participated in an in-house lecture series on terrorism and foreign relations that has featured Henry Kissinger, Jordan’s King Abdullah II, retired General Barry McCaffrey and Lawrence Wright, author of “The Looming Tower: Al-Qaeda and the Road to 9/11.”