But Mr. Mulvey said that in most cases the federal agents did not seem to have good reason to believe gang members would be present. They repeatedly passed up the invitation to check a list of 96 deportable gang associates active in Nassau County against a local police database that is updated daily, he said. They also broke promises to share a list of targets, and failed to attend a preoperation briefing, he added.

“You have to have some reason to believe the target will be there when you enter a home,” Mr. Mulvey, said. “When you have 96 warrants and you only find six of them, it’s hard to make the argument that you had a good faith basis to enter those houses.”

Mr. Mulvey said that his department had agreed to have uniformed officers present at the raids in case they resulted in local drug or weapons charges. They did not. He said he withdrew all support on Thursday, before a third night of raids, vowing not to join future operations. In contrast, Mr. Mulvey said, the department conducted a smooth and successful antigang operation with the Federal Bureau of Investigation in April. Nassau has the lowest crime rate in the nation for a county of its size, Mr. Mulvey said, in part because the police have good cooperation from the community. The conduct of the raids could undermine that relationship, he added.

“I was misled,” Mr. Mulvey said. “In good conscience, I can’t continue to cooperate unless these problems are ironed out.” Among the problems, he wrote in a Sept. 27 letter to Joseph A. Palmese, resident agent in charge of the immigration agency’s Office of Investigations in Bohemia, N.Y., was that the Nassau police could not even get a list of the 40 people arrested Monday night, leaving local officials unable to deal with a deluge of missing persons reports and inquiries from churches whose members had disappeared. Only three of the 40 were gang members, he said.

By then, he added, complaints were coming not only from immigrant advocates, but from his own officers who had seen what they said was the undisciplined conduct of the federal agents.

“These are my people that came to me,” he said in an interview after the news conference held at the Nassau County Police Department in Mineola. “These are aggressive gang enforcement investigators. Day in, day out, they work the streets, arresting gang members, and it’s unusual for them to come to me like that.”

In Suffolk County, the police commissioner, Richard Dormer, expressed complete support for the operation.