Slowly, and largely under the radar, a growing number of local law enforcement agencies across the country have moved into what had previously been the domain of the F.B.I. and state crime labs — amassing their own DNA databases of potential suspects, some collected with the donors’ knowledge, and some without it.

And that trend — coming at a time of heightened privacy concerns after recent revelations of secret federal surveillance of telephone calls and Internet traffic — is expected only to accelerate after the Supreme Court’s recent decision upholding a Maryland statute allowing the authorities to collect DNA samples from those arrested for serious crimes.

These local databases operate under their own rules, providing the police much more leeway than state and federal regulations. And the police sometimes collect samples from far more than those convicted of or arrested for serious offenses — in some cases, innocent victims of crimes who do not necessarily realize their DNA will be saved for future searches.

New York City has amassed a database with the profiles of 11,000 crime suspects. In Orange County, Calif., the district attorney’s office has 90,000 profiles, many obtained from low-level defendants who give DNA as part of a plea bargain or in return for having the charges against them dropped. In Central Florida, several law enforcement agencies have pooled their DNA databases. A Baltimore database contains DNA from more than 3,000 homicide victims.