“She asked them, ‘How many of you all think we should have a patterns and practices review investigation?’ ” Mr. Cummings recalled in an interview Thursday. “If I remember correctly, all of them raised their hands; there were about 40 of them. And I raised mine too.”

Mr. Cummings said that even before that meeting, he and other members of Congress from Maryland had a conference call with Ms. Lynch shortly after she took office in which he asked for such a review.

The decision by the Justice Department was welcome news to civil rights advocates who had been pressing for a review for a long time. “A range of people and organizations have been asking for this for years,” said Sonia Kumar, a staff attorney with the American Civil Liberties Union of Maryland, “but really, I think those calls became louder and more forceful in the wake of the death of Freddie Gray.”

Civil rights investigations often end with court settlements and independent oversight of police departments. They can be powerful agents of change, but they are not immediate, and the Baltimore investigation could take a year or more. A similar investigation into the Police Department in Ferguson, Mo., took seven months, an extraordinarily fast timeline for such cases.

Mr. Batts and the mayor had already asked the Justice Department’s community-policing experts to conduct a voluntary review of the department. The preliminary results of that review will most likely be released in the coming weeks and are expected to recommend changes to training and use-of-force policies. Those recommendations would not be binding, but Mr. Batts said he planned to work with the community-policing experts to make changes to the department regardless of what civil rights investigators did.