On Monday, Mr. Booker and Mr. McCarthy cited improvements they said they had made in addressing public concerns, like ensuring that citizen complaints are documented and investigated, rather than ignored, as large numbers of them used to be.

Deborah Jacobs, executive director of the state civil liberties group, said that the city had “taken a few baby steps in the right direction,” but that the overall situation had not improved noticeably. And with the department stretched thinner, she said, misconduct may spread, as officers work under heavier burdens and lighter supervision.

Reporters pressed Mr. Fishman and Mr. Perez several times to cite specific cases that warranted federal investigation, but they declined. Mr. Fishman said that a preliminary inquiry had “resulted in information that we thought needed to have certain follow-up.” But he would not elaborate, and he gave only a broad description of what the inquiry would cover.

“The areas will include whether officers of the Newark Police Department have engaged in a pattern or practice of excessive force against civilians, whether they have engaged in a pattern of unlawful stops, frisks, searches or arrests, and whether there has been discriminatory policing,” he said.

Similar federal investigations — civil, not criminal — have occurred in other cities, including New Orleans and Seattle, and resulted in a range of recommendations to change police practices.

In its report last year, the civil liberties union documented 38 police misconduct lawsuits that Newark settled from the start of 2008 to mid-2010, paying some $5 million, and it found 51 new suits filed against the police in the same period. The suits came not just from civilians, but also from officers claiming they had been mistreated based on race, sex or even political alliances.

One case that was settled charged that officers had used excessive force against several people, including a woman who was videotaping them, and then threatened or tried to bribe people into dropping their complaints. In another, a man who had been arrested said an officer had beaten him severely for telling another man, who was being held in the same jail, where to file an internal affairs report about his treatment. Another charged that a man died after officers, who had beaten him in an altercation, refused to call for medical help.