ST. LOUIS — A federal investigation will look into possible civil rights violations by police in the St. Louis area in the two months since protests broke out after a white former police officer was acquitted in the shooting death of a black suspect.

The U.S. Department of Justice announced the investigation in a statement Monday. The FBI and the U.S. attorney’s office in St. Louis also will help with the investigation. The statement did not specify which police agencies beyond St. Louis city will be examined and Justice Department spokesman Devin O’Malley declined comment. St. Louis County police spokesman Shawn McGuire said his department has not been informed that it is part of the investigation.

Protest leaders, Democratic U.S. Rep. Lacy Clay and others called for a federal investigation. More than 300 people have been arrested at protests since Sept. 15, when a judge ruled that Jason Stockley was not guilty of first-degree murder in the 2011 death of Anthony Lamar Smith.

Several of those arrested have alleged heavy-handed tactics and even taunting by police. Protests have occurred in multiple law-enforcement jurisdictions throughout the St. Louis region.

The Rev. Darryl Gray, a protest leader who is among those who have been arrested, said he wasn’t optimistic that the investigation will do any good after what he called decades of mistreatment of blacks by St. Louis-area police.

“Blacks have lived daily under the threat of police violence and misconduct,” Gray said.

Two of the protests drew large numbers of arrests: About 120 people were arrested in downtown St. Louis on Sept. 17, and 22 were arrested at the St. Louis Galleria shopping mall in the St. Louis County town of Richmond Heights on Sept. 23.

Journalists and other onlookers have been among those arrested. St. Louis police used a tactic known as “kettling” that forces people into an increasingly smaller area during the Sept. 17 arrests, which occurred after protesters hurled rocks and other objects at police and broke out windows at several downtown businesses.

Some of those arrested said they wanted to leave but couldn’t because of the kettling. A lawsuit filed Sept. 22 by the American Civil Liberties Union of Missouri alleges that St. Louis police shoved the heads of some of those arrested into the pavement and taunted protesters by chanting “Whose streets? Our streets!” a common chant by demonstrators.

The allegations prompted interim St. Louis police Chief Lawrence O’Toole and the city’s mayor, Lyda Krewson, to ask for a federal investigation on Sept. 27.

Krewson said in a statement Monday that they are pleased the investigation is underway and appreciate the “work and professionalism” of the U.S. attorney’s office in St. Louis. Police spokeswoman Schron Jackson said O’Toole “welcomes the independent review from outside agencies, and is committed to assisting them in any way needed.”

U.S. Rep. Clay requested an investigation of St. Louis police last week in a letter to the Justice Department. On Monday, he issued a statement saying, “The First Amendment is vital, because like our local police, it defends everyone. Both are essential to our country and our community.”

Those arrested at the Galleria on Sept. 23 included a 13-year-old boy and the Rev. Kayla Frye of St. Peter African Methodist Episcopal Church in St. Louis. Frye was charged with assault for allegedly jumping on a police officer’s back. Several elected officials and faith leaders called the altercation a “police riot,” but police said the arrests were necessary after protesters became violent.

Last week, U.S. District Judge Catherine Perry issued a temporary restraining order prohibiting St. Louis city police from shutting down nonviolent protests or using chemical agents such as mace as a way to punish demonstrators. But the ruling applied only to city officers.

Stockley, the ex-officer who was acquitted, had tried with his partner to corner Smith in December 2011 after observing what they thought was a drug transaction on a fast food parking lot. Smith drove away, nearly striking the officers. That led to a chase.

Smith, 24, was fatally shot by Stockley at the end of the chase. Stockley testified he thought Smith was reaching for a gun that was found inside Smith’s car. Prosecutors alleged that Stockley planted the weapon.

Stockley, who left the police force in 2013, was acquitted after a trial that was decided by a judge. Stockley waived his right to a jury trial.