The technology, often known by the brand name StingRay, is a rectangular device about the size of a suitcase that essentially tricks nearby cellphones by acting like a cellphone tower and intercepting the phone’s signal. The devices can also capture texts, calls, emails and other data.

“Our cell-site simulator technology does not allow us to intercept what people are saying to each other,” Mr. Byrne said. He said that meant the police did not need an eavesdropping warrant to use the devices. And in any case, he said, the department a couple years ago instituted a new policy under which it already shows probable cause on applications to use cell-site simulators, going beyond the usual requirements of the order.

Mr. Greco said based on applications the Legal Aid Society has reviewed, the department uses a lower standard of reasonable suspicion, rather than showing probable cause.

Because civil rights lawyers say prosecutors in New York almost never acknowledge using the devices, defendants are typically in the dark and defense lawyers are kept from raising challenges.

Some states, like California, have tried to force police agencies to seek approval from their city councils before using the devices. In Baltimore, where the police have used cell-site simulators thousands of times, defense lawyers have pressed for details about how the police found certain suspects, only to have prosecutors drop charges to avoid answering questions. And the Department of Justice now requires federal agents, with a few exceptions, to obtain a warrant from a judge before using the devices.

Their use is increasing. The American Civil Liberties Union has identified 72 law enforcement agencies, in 24 states and the District of Columbia, that use cell-site simulators.

In New York, the Legal Aid Society has trained lawyers to spot the signs that a cell-site simulator was used: a suspect’s cellphone suddenly dying, or a sudden arrest in a place investigators had not previously identified as of interest. The Legal Aid Society said the Brooklyn case was the first it knew of in which prosecutors in New York City told defense attorneys about the approval to use a cell-site simulator, a fact that they said only emerged after much prodding.