Public records and interviews show that Mr. Sessions’s new direction is also having a chilling effect on potential new consent decrees, even in places where there is local support for them. The department declined to push for one in Chicago, and in the small Acadian city of Ville Platte, La., it appears to have dropped Obama-era efforts to impose one.

In 2014, an F.B.I. agent assisting with a murder investigation in Ville Platte stumbled onto a broader problem: The police and sheriff’s deputies there had a habit of throwing people in jail without probable cause or a warrant. The detainees — including witnesses who were not suspected of a crime — were held for days and questioned without access to a lawyer, a phone or even, sometimes, toothpaste and tampons.

The police chief, Neal Lartigue, acknowledged that the detentions, known as 72-hour holds, had been going on for as long as he could remember, and said that he had not known they were unconstitutional.

The F.B.I. agent, Steven Krueger, alerted numerous agencies to his findings, according to interviews with state and local law enforcement officials who recounted the previously unreported events. But his effort stalled: The local district attorney turned out to have been personally involved in the illegal interrogations, the state attorney general at the time declined to take the case, and the state inspector general investigated but did not have the power to prosecute.

The case fell to the Justice Department’s civil rights division.

In a report issued in the waning days of Mr. Obama’s tenure, the civil rights division said the Ville Platte police and the Evangeline Parish Sheriff’s Office had illegally detained at least 900 people in a three-year period.

In the Obama administration, such a report typically would have been followed by a consent decree or other court-enforceable agreement, in which the police would agree to carry out specific reforms — most likely, in Ville Platte’s case, in the areas of training officers and rebuilding trust. Local officials in Ville Platte said they were still awaiting a proposed agreement from the Justice Department, but added that they had fully cooperated with the investigation, which, after all, was instigated by a law enforcement official.