So-called homegrown terrorist cells are as dangerous to America’s safety as international groups such as Al Qaeda, a report released yesterday by the New York City Police Department said.

The report by senior NYPD intelligence officials, available for download here, outlines how “unremarkable” people living in Western countries have been transformed into terrorists, and it calls the September 11, 2001, attacks an “exception” to the usual pattern.

“The people who take this path come from all walks of life,” the police commissioner, Raymond Kelly, 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, which he commissioned, is a “groundbreaking” new tool that law enforcement agencies can use in investigating and intercepting locally hatched terrorist plots.

The report is based on the stories of more than 100 Muslim terrorists from six countries who were involved in 11 past plots, including September 11, which were studied to create a “road map” of radicalization.

Most often, the report said, Al Qaeda provided little more than inspiration to the groups studied, which include the pair of New York residents who plotted to attack the Herald Square subway station in 2004. However, 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.

The report, which is focused exclusively on Muslim groups, traces how individuals were transformed from ordinary people to terrorists willing to kill hundreds of people. It points to telltale signs that appeared in each of the cases, such as an identity crisis and an intensifying 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 the police identify are often highly personal and difficult to detect — for example, 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, an assertion that is 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 Council on American-Islamic Relations, Parvez Ahmed, criticized the report as 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 one of the report’s examples, 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,” he said. “We make certain that everything we do certainly adheres to the Constitution.”

The chairman of the Senate Homeland Security Committee, Senator Lieberman of Connecticut, 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 committee this fall.

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

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Read the NYPD report.