The Justice Department and the FBI have opened an investigation into “allegations of potential civil right violations by law enforcement officers” in St. Louis, Missouri, according to federal prosecutors, a development that follows outcry over the treatment and arrest of demonstrators protesting a police officer’s acquittal of murder charges.

Jeff Jensen, the newly sworn in U.S. attorney for the Eastern District of Missouri, said his office is leading the investigation and will work with the FBI and the Justice Department’s Civil Rights Division to review law enforcement officers’ actions during a Sept. 15 protest, as well as demonstrations that took place in the weeks that followed.

Local leaders, including the St. Louis mayor and the police chief, previously had asked the U.S. Attorney’s Office for an independent, third-party investigation of police actions, as has the NAACP Legal Defense and Educational Fund.

Mr. Jensen’s announcement of the probe comes a little more than a month after he was sworn into office and a week after a federal judge found that St. Louis police officers had acted against protesters in an “arbitrary and retaliatory fashion.”

The protests arose from the Sept. 15 acquittal of white former St. Louis police officer Jason Stockley, who had faced murder charges in the 2011 fatal shooting of Anthony Lamar Smith, a 24-year-old black man suspected of drug crimes.

Police and protesters have faced off numerous times in the St. Louis area since then, with at least 300 people arrested during recent demonstrations.

The American Civil Liberties Union of Missouri filed a class-action lawsuit accusing police of violating demonstrators’ civil rights during a Sept. 17 protest in which 123 people were arrested, saying officers didn’t warn protesters to disperse.

The lawsuit says police forced witnesses to stop videotaping and used pepper spray on people who were restrained.

U.S. District Judge Catherine D. Perry issued a preliminary injunction last week that curtails some police practices in the city, including use of chemical agents against individuals who are engaged in nonviolent expression of their First Amendment rights. Police also are banned from declaring an “unlawful assembly” unless there is an imminent threat of violence.

Tony Rothert, legal director of the ACLU of Missouri, said it is important that the Justice Department investigate “the systemic violation of civil rights by the St. Louis Metropolitan Police Department.”

But St. Louis city officials also should “proactively engage with the community now to develop a collaborative policing model that protects constitutional rights and promotes public safety,” Mr. Rothert said.

Todd Cox, police director of the NAACP Legal Defense Fund, called Monday’s development a “first step” and promised the organization would monitor how the probe unfolds.

“The volume of complaints highlights the need for more systemic policing reform in St. Louis to ensure such police violence does not occur again,” Mr. Cox said.

With the U.S. attorney’s investigation ongoing, Mr. Jensen declined to comment about the scope of the investigation or to provide other details.

The investigation comes as Attorney General Jeff Sessions has attempted to retreat from the previous administration’s more aggressive use of court-mandated agreements to monitor local law enforcement — a process he said maligns departments for the bad behavior of a few officers.

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