So-called homegrown terrorist cells are as dangerous to America’s safety as outside groups such as Al Qaeda, a report released by the police department today said.

The report by senior NYPD intelligence officials, available for download here, outlines how “unremarkable” people living in Western countries were transformed into terrorists, calling the September 11, 2001 attacks an exception.

“The people who take this path come from all walks of life,” the police commissioner, Raymond Kelly, who commissioned the report, said. “Their lives are not marked by deprivation. They don’t stand out.”

Although homegrown terrorism is not a new concept, Mr. Kelly said the report is a “groundbreaking” new tool that law enforcement agencies can use in investigating and intercepting locally hatched terrorism plots.

The report is based on the stories of more than 100 Muslim terrorists from six different countries involved in 11 plots in recent years, including September 11. The subjects provide a “road map” of radicalization.

Most often, the report said, Al Qaeda provided little more than inspiration to the groups studied, which included the pair of New York residents who plotted to attack the Herald Square subway station in 2004. The authors, an NYPD senior intelligence analyst, Mitchell Silber, and an NYPD intelligence research specialist, Arvin Bhatt, said that with September 11 as a backdrop radical ideologies are proliferating in Western countries.

“The precedent has been set that you can fulfill jihad in your own country,” the NYPD’s assistant commissioner for intelligence, Lawrence Sanchez, said.

In the report, which focused only on Muslim groups, the authors trace how individuals transformed from ordinary citizens to terrorists willing to kill hundreds of people. They pointed to telltale signs that appeared consistently in each of the cases, such as an identity crisis and an intensification of religious faith.

The report also identified the Internet as the new terrorist training ground. “We say that the Internet is the new Afghanistan,” Mr. Kelly said.

The problem for law enforcement, the report said, is that the steps to radicalization are often highly personal and difficult to detect — an individual might grow a beard, stop wearing baseball caps, and smoking cigarettes, or gradually cut off contact with family members.

“There is no useful profile to assist law enforcement or intelligence to predict who will follow this trajectory of radicalization,” the report said.

Still, the authors identified individuals most likely to follow the radicalization path: male, middle-class Muslims in their 20s and 30s, raising hackles among civil liberties and Arab-American groups.

“The NYPD must have the tools it needs to investigate and combat terrorism, but this report lays the foundation for blanket surveillance of the entire Muslim community,” the associate legal director for the New York Civil Liberties Union, Christopher Dunn, said. “Making all Muslims suspects is ethnic profiling, and it’s unconstitutional.”

The chairman of the National Council on American-Islamic Relations, Parvez Ahmed, criticized the report for being contradictory.

“While labeling almost every American Muslim as a potential terrorist, the report’s authors admit that their findings offer no useful way to identify real terror suspects,” he said, noting that giving up cigarettes should be regarded as a good thing.

Mr. Kelly was emphatic that the report is not about “stereotyping.”

“We’re very sensitive to those issues,” Mr. Kelly said. “We make certain that everything we do certainly adheres to the constitution.”

The Senate Homeland Security chairman, Joe Lieberman, called the report a “breakthrough in our efforts to defend our homeland,” and said he would call on the authors to testify in front of the Senate this fall.

Mr. Kelly added that the report, which has also been presented to federal agencies including the Department of Homeland Security and the CIA, would not lead to more surveillance, but that instead would change how police think about terrorism investigations.

“You have to know that there is a process,” Mr. Kelly said. “So you at least have the potential for stopping it.”

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Read the NYPD report.