The catalyst was a source's cryptic hint: Ask about Ray Kelly's rakers and mosque crawlers. Their curiosity piqued, two Associated Press investigative reporters began digging.

As a result, The Associated Press won the Pulitzer Prize for investigative reporting Monday for stories revealing that the New York Police Department _ headed by Commissioner Raymond Kelly _ had built an aggressive domestic intelligence program after the Sept. 11 attacks that put Muslim businesses, mosques and student groups under scrutiny.

In a series of articles that began in August, investigative reporters Matt Apuzzo and Adam Goldman, along with colleagues Eileen Sullivan and Chris Hawley, documented that police had systematically spied on Muslim neighborhoods, listened in on sermons, infiltrated colleges, and photographed law-abiding residents as part of a broad effort to watch communities where a terror cell might operate. Individuals and groups were monitored even when there was no evidence they were linked to terrorism or crime.

The investigation revealed that Kelly had brought in a CIA official to help develop an intelligence division unlike that of any other U.S. police department. It assigned "rakers" to ethnic neighborhoods, infiltrating enterprises ranging from booksellers to cafes, and "mosque crawlers" to Muslim houses of worship.

The four winners were toasted by scores of colleagues gathered in the newsroom of the AP's New York headquarters.

"We kept reporting things that no one in the city of New York knew about," said the AP's executive editor, Kathleen Carroll. "That's what I'm most proud of."

The tactics disclosed by the series stirred debate over whether the NYPD was infringing on the civil rights of Muslims and illegally engaging in religious and ethnic profiling. Hundreds of Muslims staged rallies to protest the spying, and the disclosures prompted more than a dozen religious leaders to boycott Mayor Michael Bloomberg's annual interfaith breakfast.

In Washington, 34 members of Congress demanded a federal investigation into the NYPD's actions. Attorney General Eric Holder said he was disturbed by reports about the operations, and the Justice Department said it was reviewing complaints received from Muslims and their supporters.

The AP's reporting also prompted an investigation by the CIA's inspector general. That internal inquiry concluded that the CIA, which is prohibited from domestic spying, hadn't broken any laws, but criticized the agency for allowing an officer assigned to the NYPD to operate without sufficient supervision.

It was while reporting on an unrelated story about CIA officers' misconduct in 2010 that Apuzzo and Goldman first got wind of the NYPD program through hints from their sources.

Over the next eight months, Apuzzo, Goldman and their colleagues conducted dozens of interviews and delved through hundreds of secret documents to uncover the vast surveillance operation. Their editors, said Goldman, "never told us to stop. They always asked `What's next?'"

Reaction to the stories from some quarters in New York was harsh. Kelly claimed that the AP had mischaracterized the intelligence-gathering program. The city's two tabloids, The Daily News and The New York Post, published editorials defending the NYPD and criticizing the AP series. Bloomberg said he was committed to preventing another terror attack, even if it meant keeping a close eye on law-abiding Muslims.

"We came under relentless attack," Goldman told his colleagues celebrating the prize. "Some people thought they could intimidate us and the AP _ and they were wrong."

Among the many disclosures in the series:

_Even as Bloomberg dined with local imam Reda Shata, and as The New York Times chronicled Shata's efforts to reconcile Muslim traditions with American life, the NYPD had him under surveillance.

_The police infiltrated Muslim student groups at schools like Brooklyn College, checking on Internet activity and worrying over "militant paintball trips" that the students insist were recreational. To get access to student records, officers told campus police they were investigating narcotics or gang cases.

_A Moroccan Initiative catalogued the lives of Moroccans in the city, monitoring them at restaurants, grocery stores, barbershops. Sometimes, they were interviewed by police officers who said they were conducting criminal investigations or looking for a lost child.

The articles have continued at a steady pace this year, detailing how the NYPD monitored Muslims beyond the city limits.

New Jersey Gov. Chris Christie accused the NYPD of acting like "masters of the universe" by sending agents into his state. Mayor Cory Booker of Newark _ New Jersey's largest city _ also complained, and the FBI chief in New Jersey said the surveillance undermined the bureau's own efforts by sowing distrust of authorities among Muslims.

Schools such as Yale, the University of Pennsylvania and the University of Buffalo condemned surveillance aimed at their students. One document revealed that the NYPD sent an undercover officer on a whitewater rafting trip with Muslim students from the City College of New York.

The Pulitzer was the 50th for the AP, a frequent winner in news photography categories. The AP's most recent previous Pulitzer for news writing was in 2000, for a series of stories about a massacre of civilians by U.S. troops during the Korean War.

The Pulitzer is the fourth major award for the NYPD series, which also won the Goldsmith Prize for Investigative Reporting, a George Polk Award, and the Edgar A. Poe Award from the White House Correspondents' Association.

Goldman, 40, joined the AP in 2002. Apuzzo, 33, was hired in 2003. Sullivan, 35, has been with the AP since 2007. Hawley, 39, recently left the news agency to take a job with an aerospace company. During more than 20 years in journalism, he had two stints at the AP, totaling eight years.

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Highlights of the AP series: http://www.ap.org/media-center/nypd/investigation