WASHINGTON — Attorney General Eric Holder on Thursday formally opened a broad civil rights investigation into the Ferguson, Mo., Police Department, as well as a cooperative reform effort with the St. Louis County Police Department.The federal probe that follows the Aug. 9 shooting of unarmed teenager Michael Brown could take months, if not years, as investigators peer into whether the nearly all-white Ferguson department has a “pattern or practice” of discriminatory behavior. Further reforms could follow, enforced through lawsuits if necessary.“We will follow the facts and the law wherever they may lead,” Holder said at a news conference. “And if, at any point, we find reason to expand our inquiry to include additional police forces in neighboring jurisdictions, we will not hesitate to do so.”The Ferguson investigation will focus on four primary areas, according to Molly Moran, acting assistant attorney general for civil rights. These include the Ferguson department’s use of force; stop, search and arrest practices; treatment of those in detention; and any other potentially discriminatory practices that come to light. “Over the last several weeks, we’ve received a lot of information from residents,” Moran said.Since Brown’s shooting, a sometimes harsh light has been cast on the relations between the Ferguson Police Department, three of whose 53 commissioned officers are African-American, and the town’s residents, 67 percent of whom are African-American.Last year, African-Americans drove 86 percent of the cars stopped by Ferguson police officers, according to the state’s annual racial profiling report. Once pulled over in Ferguson, African-American drivers were twice as likely to be searched, according to the report.The “pattern-or-practice” investigation opened Thursday is different from an ongoing federal inquiry into the shooting itself. The federal shooting probe theoretically could lead to civil rights charges against Officer Darren Wilson, the Ferguson policeman who shot Brown. That would be separate from any potential state prosecution, although a federal criminal case would face high burden-of-proof hurdles.Holder, who visited Ferguson two weeks ago, said Justice Department officials met Wednesday with Ferguson and St. Louis County officials and have been promised full cooperation.“We’ve done a good, solid job,” Ferguson Police Chief Thomas Jackson told NBC News on Thursday, but “we can be better, and we’re willing to be.”With St. Louis County, the Justice Department’s Office of Community Oriented Policing Services will be conducting assessments, completing an “after-action report” on police reactions to the Ferguson protests, and offering assistance, including training for both new and experienced officers.“This is a victory, but there is much work to be done,” Tef Poe, the St. Louis-based organizer of an activist group called Hands Up United, said in a statement.Amnesty International USA executive director Steven W. Hawkins also called news of the investigation “welcome,” while Sen. Claire McCaskill, D-Mo., called it a “step in the right direction.”The pattern-or-practice investigations are handled by the Special Litigation Section, part of the department’s Civil Rights Division. Holder has initiated more than 20 of these investigations over the past five years, more than twice as many as were opened in the last five years of the Bush administration.In November 2013, for instance, an investigation was opened into the Family Court of St. Louis, following allegations that it was violating the due process rights of juveniles. The probe has not yet resulted in public findings.Other pattern-or-practice investigations have targeted the Albuquerque Police Department in New Mexico, the Miami Police Department in Florida and the Newark Police Department in New Jersey, among others.The investigations can take years.In May 2011, for instance, federal officials opened their Newark investigation. The final report was issued in July 2014, finding police had routinely violated residents’ constitutional rights. The Albuquerque investigation that opened in November 2012 also lasted until this year.These broader investigations can incite federal lawsuits, as happened when the Justice Department sued the state of Mississippi, the city of Meridian and Lauderdale County in October 2012 over the treatment of juveniles. Settlement talks were still underway as recently as August, court records indicate.The investigations also can incite collaboration on systemic reforms. The Pennsylvania Department of Corrections, for instance, instituted improvements following a Justice Department inquiry into the dangers facing gay and transgender inmates.“With these agreements, we have seen dramatic decreases in excessive uses of force, greater equity in the delivery of police services . . . and, most significantly, increased confidence by communities in their law enforcement agencies,” Holder said.The Special Litigation Section is headed by attorney Jonathan M. Smith, an Antioch College of Law graduate who formerly headed the Legal Aid Society of the District of Columbia and, before that, the D.C. Prisoners’ Legal Services Project.