The department has been working on the program for months, Mr. Browne said. But the effort caught many in the city’s legal circles by surprise as news of it began trickling out late last week. It is raising concerns among civil libertarians and privacy advocates, who say the authorities’ cataloging of the new data could put innocent people under permanent suspicion.

“It’s really distressing that the Police Department is once again undertaking a new regime of personal data collection without any public discourse,” said Donna Lieberman, the executive director of the New York Civil Liberties Union, “and we don’t know the reason for it, whether this is a necessary program, whether it’s effective to address the concerns that it’s designed to address, and whether in this day and age it’s even cost-effective, not to mention whether there are any protections in place against the misuse of the data that’s collected.”

Steven Banks, attorney in chief of the Legal Aid Society, said his office learned about the program on Friday in a phone call from the mayor’s criminal justice coordinator.

“This is an unnecessary process,” Mr. Banks said. “It’s unauthorized by the statutes and of questionable legality at best. The statutes specifically authorize collecting fingerprints. There has been great legislative debate about the extent to which DNA evidence can be collected, and it is limited to certain types of cases. So the idea that the Police Department can forge ahead and use a totally new technology without any statutory authorization is certainly suspect.”

Mr. Browne said a legal review by the department had concluded that legislative authorization was not necessary.