And many who have been victims of police violence have turned to the DOJ for help from departments they believe are not working in their best interest.
And now Sessions wants to postpone a federal court hearing set for this week that would have allowed citizens of Baltimore to speak out about their embattled police force.
What does this mean, and why does it matter?
If federal investigators uncover such a "pattern or practice," the Justice Department can negotiate a deal with the police force to change its ways, or -– if the police force won't cooperate -- the DOJ can file a federal lawsuit asking a court to force changes within the police agency.
The Justice Department opened twice as many "pattern or practice" investigations under President Obama as it did under President George W. Bush.
Of the more than 20 cases opened under the Obama administration, most have now been resolved through agreements or settlements -– known as "consent decrees."
Consent decrees often give the Justice Department authority to direct specific changes and initiatives within police departments, and agree to let judges oversee and enforce those changes.
A SKEPTICAL SESSIONS
Sessions has long been skeptical of federal investigations into police departments accused of biased policing.
During his confirmation hearing to become attorney general in January, Sessions said, "There is concern that good police officers and good departments can be sued by the Department of Justice when you just have individuals within a department that have done wrong."
And only days after being sworn in as attorney general in February, Sessions criticized reports by his own agency that detailed "systemic" problems with police in Chicago and Ferguson, Missouri, where separate killings of unarmed black men sparked protests across the country in 2014.
While acknowledging he never read the actual reports, he called parts of them "pretty anecdotal and not so scientifically based."
More than a decade earlier, while serving as a senator from Alabama in 2002, Sessions warned a top Justice Department official that, "Just because someone says it's [a matter of] civil rights, maybe they haven’t done their homework. Maybe they haven’t studied the facts or researched the laws quite enough."
'REVIEW' AND RESCHEDULE
On Friday, Sessions issued a memo to top Justice Department officials, directing them "to immediately review all Department activities – including … existing or contemplated consent decrees."
Among the consent decrees being contemplated is a deal with the Baltimore Police Department.
Federal and city attorneys negotiated a draft agreement last year to bring major changes to the police force, after the Justice Department released a scathing report documenting a history of excessive force and a pattern of "unconstitutional stops, searches, and arrests" that disproportionately targeted African Americans.
A federal judge is still deciding whether to sign off on the agreement, and a hearing was scheduled for Thursday to allow members of the public to weigh in on the matter. But on Monday night, the Justice Department filed a motion asking the judge to postpone the hearing until at least July.
Nevertheless, the motion said federal officials are "aware of the need for police reform in Baltimore and of the need to rebuild public confidence in law enforcement in Baltimore."
The Baltimore Police Department -– the very target of the Justice Department's investigation -– opposes the Justice Department's latest move.
"Further delays only serve to erode the trust of the public in this process," a Baltimore Police Department spokesman said in a statement. "The Baltimore Police Department is continuing to move forward with reforms related to the forthcoming consent decree for the overall progress of the city of Baltimore."
The mayor of Baltimore agreed, saying city officials "strongly oppose any delay in moving forward."
"I, along with Police Commissioner Kevin Davis and the citizens of Baltimore, recognize that reforming our police department is long overdue," Mayor Catherine Pugh said in a statement. "Much has been done to begin the process of building faith between the police department and the community it seeks to serve. Any interruption in moving forward may have the effect of eroding the trust that we are working hard to establish."
The mayor of Chicago, Rahm Emanuel, also issued a statement, saying he "can't speak for the federal government's" intentions.
"The reforms we have made over the past year are built on the principles of partnership and trust between our residents and our officers, and they laid the foundation for the 2017 reform plan we outlined just a few weeks ago," Emanuel said. "Through these ongoing reforms we will ensure our officers have the training and support they need to do a tough job well, we will strengthen the relationship with our residents, and we will make our city a stronger, safer place. Reform is in our self-interest and that is why Chicago has been, is, and always will be committed to reform."