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Chief Lanier ensured Mr. Mendelson that the Metropolitan Police Department “does not have unlimited access to electronic communication.”

“Working with prosecutors, we seek warrants or subpoenas for electronic records,” she said. “Not only is this our normal practice, but service providers also have a vested interest in preserving the privacy of their customers to the greatest extent allowable.”

The officials also had serious concerns about other parameters of the proposal, including the time frame in which a “subscriber” — the person whose information is being sought — would be notified of the request from law enforcement to the third party that controls the server containing the emails.

On the other side of the debate, the D.C. chapter of the American Civil Liberties Union and the Computer and Communications Industry Association sent letters of support to Mr. Catania, citing their concerns about privacy and unbridled government access in the digital age.

On Wednesday, he said the council should be able to forge a bill despite opposition from law enforcement.

“Make no mistake —law enforcement wants to make it easier to do their job, not harder,” he said. “Our job is to counterbalance that to protect the civil liberties of our citizens.”

Other city lawmakers said they were intrigued by Mr. Catania’s proposal, but they want to host a healthy debate between law enforcement and those who tout individuals’ privacy rights.

Council member Marion Barry, Ward 8 Democrat and a former mayor who was famously arrested in an FBI sting on crack cocaine charges in 1990, called on the council to strike the right balance when it comes to the government’s surveillance of citizens.

“These things can be done,” he said of the tactics. “Nobody knows they’re being done — I’m certainly a witness to that.”