The investigation was requested by the city's mayor, and is expected to be announced officially on Friday, the source said.
The investigation comes in the wake of Freddie Gray's death, and the outrage that followed.
Gray, 25, suffered a fatal injury to his spine while in police custody under circumstances that remain unclear. Six officers have been arrested and face various charges, ranging from false imprisonment to murder.
In asking for the investigation, Baltimore Mayor Stephanie Rawlings-Blake said such an inquiry was necessary to achieve the kind of "sustainable and significant reform" of the department that she and the citizens of Baltimore want to see.
"The DOJ has employed these investigations in communities across our nation to reform serious patterns and practices of force, biased policing and other unconstitutional practices by law enforcement," she told reporters. "I'm asking the Department of Justice to investigate if our police department has engaged in a pattern or practice of stops, searches or arrests that violate the Fourth Amendment."
The Justice Department is already investigating whether Gray's civil rights were violated. But the so-called pattern or practice investigation would be far broader, a full-scale civil rights investigation of the entire police department.
The inquiry would be similar to the one conducted in Ferguson, Missouri, after the shooting death of Michael Brown last summer.
The Justice Department's investigation there found systematic discrimination against African-Americans in that city by the police department and the municipal court system, with blacks subject to far more vehicle stops, citations and arrests than their proportion of the population.
Blacks were more likely to be searched during traffic stops and cited for minor infractions, and 88% of the cases in which police used force were against African-Americans, who make up 67% of the population of Ferguson.