NASHVILLE (AP) — A district attorney in Tennessee said Thursday that his office has dropped 30 to 40 cases involving the five former officers who have been charged with second-degree murder in the beating death of Tyre Nichols.

In a statement, Shelby County District Attorney Steven Mulroy's spokesperson also said charges were reduced in about a dozen other cases involving the former Memphis Police officers. Four cases were referred to the U.S. attorney's office for allegations of excessive force, the district attorney's office added.

The decisions follow a review by Mulroy's office of about 100 cases shared among the officers.

"DA Mulroy cites that the dismissals came down to the lack of credibility from the five officers since the charges," Mulroy's spokesperson, Erica Williams, said in the statement.

Caught on police video, the beating of the 29-year-old Nichols was one in a string of violent encounters between police and Black people that sparked protests and renewed debate about police brutality and police reform in the U.S.

Five officers have pleaded not guilty to criminal charges including second-degree murder in the Jan. 7 beating of Nichols after a traffic stop — and his death three days later.

The five officers — Tadarrius Bean, Demetrius Haley, Emmitt Martin, Desmond Mills and Justin Smith — charged in the case are Black. So was Nichols.

The officers were part of a crime-suppression team known as Scorpion. They punched Nichols, kicked him and slugged him with a baton as he yelled for his mother. Memphis Police Chief Cerelyn "CJ" Davis disbanded the Scorpion unit after Nichols' death, though members of the unit have been moved to other teams.

Last month, the U.S. Department of Justice said it is investigating how Memphis Police Department officers use force and conduct arrests.

Even in the majority Black city of Memphis, the police department may be disproportionately focusing its traffic enforcement on Black drivers, Assistant Attorney General Kristen Clarke of the Civil Rights Division said in announcing the investigation.

The Justice Department announced in March a separate review concerning the use of force, de-escalation strategies and specialized units in the Memphis Police Department. Federal investigators also are looking specifically into Nichols' arrest and death. Nichols' mother has sued the city and its police chief over her son's death.