The city says the informant, who is cited in documents as male, “strenuously objects” to having his name revealed because he has known Mr. Creighton “for a very long time” and also knows the Creighton family. The informant “maintains a relationship” with Mr. Creighton and his family, the city says, and believes Mr. Creighton has “no idea” that he helped the police and that he testified before the grand jury.

“The informant fears that he would be in physical danger of retaliation if the plaintiff knew his identity,” the city said in a court filing seeking to keep the informant’s identity secret.

In the court system, the parties to a lawsuit have broad rights to gather evidence, a process called discovery, including information about probable witnesses against them. Initially, when the city refused to divulge the informant’s name, Mr. Creighton’s lawyers objected. A magistrate judge, Debra Freeman of the Federal District Court in Manhattan, then ordered that the lawyers be given the name on an “attorney’s eyes only” basis, meaning that they could not tell their client.

In April, after further arguments, Judge Freeman ruled that the name should also be given to Mr. Creighton, an order the city has appealed to Judge Paul G. Gardephe, also of Federal District Court. The order has been stayed pending his decision.

Mr. Creighton’s lawyers have said in court papers that not being able to reveal the informant’s name to their client has hamstrung them in their case. They cannot discuss the informant with their client, or what motives he might have had for incriminating him, nor can they question other people about the informant, because that would reveal his identity.