Baltimore mayor Stephanie Rawlings-Blake today called for an in-depth federal investigation into the “patterns and practices” of her city’s police department. Rawlings-Blake’s request came one day after Attorney General Loretta Lynch visited Baltimore with other Department of Justice officials in the wake of riots and protests over the death of Freddie Gray, a 25-year-old black man who died while in police custody. 

Baltimore had been engaged in reform work with federal officials, but Rawlings-Blake has called for an investigation of the kind the Justice Department pursued in Ferguson, Mo. following the death of Michael Brown, which found long-running and wide-ranging racial disparities in stops and arrests. 

“We all know that Baltimore has a fractured relationship with community,” Rawlings-Blake said, the Chicago Tribune reported. 

But a pattern or practice investigation could set off more tension between political leaders and police at a time when the newly sworn in Lynch is juggling competing political interests. Among them is Lynch’s own serious effort to distinguish herself from her predecessor, Eric Holder. Politico reported from one of Lynch’s stops in Baltimore with community and religious leaders yesterday:

While some in the room have been pressing for a larger DOJ investigation into Baltimore police practices, the attorney general held her cards close to her vest, sounding more like a counselor than a prosecutor.

“We’re here to hold your hands and provide support,” she told the group.

Within minutes, Lynch was on her way to Baltimore police headquarters, where she saluted officers as heroes, including one injured when the windshield of his police car was shattered during the violence last week.

Incendiary as the findings from DOJ pattern or practice investigations may be, they also lack subpoena power, and so investigators are limited in what they can find and the long-term change those probes can effect. The Justice Department, it turns out, can only do so much to demand and effect systemic change in the nation’s police departments.