Attorney General Loretta Lynch will launch a federal probe into whether Baltimore's police department has engaged in a "pattern or practice" of excessive force, the Washington Post reported on Thursday.

The Post, citing two law enforcement officials, said Lynch's announcement of the investigation could come as soon as Friday.

Baltimore Mayor Stephanie Rawlings-Blake asked the U.S. Justice Department on Wednesday to investigate the city's police department for civil rights violations after the death of a black man from injuries sustained in police custody.

Rawlings-Blake told a news conference the investigation would look into police practices such as frisks, street stops of suspects and arrests to see if they violated the U.S. Constitution.

Her request followed the April 19 death of Freddie Gray, 25, who sustained spinal injuries after being arrested. His death sparked protests and a day of arson and looting in the largely black city.

Lynch, who took office last week, met on Tuesday with Gray's family and Baltimore officials and vowed to help the city pursue police reform.

The city's chief prosecutor has brought criminal charges, including one murder charge, against six officers involved in the April 12 arrest of Gray.

The Justice Department has conducted similar reviews of U.S. police departments. An investigation of police in Ferguson, Missouri, where a white officer shot dead an unarmed black teenager last year, concluded in March that the department routinely engaged in racially biased practices.