The investigation into Ferguson’s policing practices will run parallel to a separate federal inquiry focused specifically on the circumstances of Mr. Brown’s shooting. In addition, President Obama — responding to disturbing images of Ferguson police officers facing down civilians with military assault equipment — has rightly ordered a review of the government’s policy of outfitting local police with that level of firepower.

The civil rights investigation will focus on whether officers there made discriminatory traffic stops, mistreated prisoners or used excessive force in the years before last month’s fatal shooting of Mr. Brown.

The Justice Department will find plenty of evidence of disparate treatment of black motorists in St. Louis County, which is crowded with municipalities. As The Washington Post reported last week, some of these towns get 40 percent or more of their revenue from traffic fines and fees from petty violations. And since there are 90 municipalities in St. Louis County, that means drivers can pass through several towns in just a few miles on one main thoroughfare. Motorists who are detained in one town are often dragged through the courts or jails in several communities.

As Campbell Robertson and Joseph Goldstein of The Times reported last month, the municipal courts in places like Maplewood, Mo., are filled with blacks who are pulled over by officers and charged with offenses that increase in cost when a defendant misses a court date. According to a report by the state attorney general in 2013, black motorists in Maplewood were searched or arrested during stops at more than twice the rate of whites — even though searches of blacks and whites were similarly likely to turn up contraband.

A startling analysis released last month by ArchCity Defenders, a nonprofit group in St. Louis, offered up a harrowing portrait of these interlocking court systems, which appear to be structured to persecute minority communities. In one community, “100 percent of all searches and arrests originating from traffic stops in Bel-Ridge in 2013 were of black individuals,” the report said.