(CNN) — Five former Memphis Police Department officers pleaded not guilty Friday at their arraignment on criminal charges connected to the January death of Tyre Nichols, whose brutal beating after a police traffic stop was seen on video.

Tadarrius Bean, Demetrius Haley, Justin Smith, Emmitt Martin III and Desmond Mills Jr. each face charges of second-degree murder, aggravated assault, aggravated kidnapping, official misconduct and official oppression.

The former officers are due back in court on May 1. Their attorneys entered not guilty pleas on their behalf.

Second-degree murder in Tennessee is considered a Class A felony punishable by 15 to 60 years in prison.

Shelby County District Attorney Steve Mulroy told CNN one challenge for prosecutors will be proving the defendants knowingly killed Nichols — which under state law means “they acted in such a way that they were reasonably certain that their actions could cause death.”

Nichols, a 29-year-old Black man, was repeatedly punched and kicked by the officers charged following a traffic stop and brief pursuit on foot on January 7. Nichols was hospitalized after the beating and died three days later.

Mulroy said his office is still looking into “people who showed up after the beating had concluded” and were “involved directly or indirectly in the death.”

“We’re going to do what we can as quickly as I can — but thoroughly — and we’ll be making decisions about charges regarding all of those people and in the time ahead,” he said.

The five officers, who are also Black, were fired following an internal investigation and were indicted on January 26.

The following day, body camera videos and surveillance footage from the arrest were released, demonstrating the severity of the beating to the public and drawing widespread condemnation from residents and police officials alike. Protesters marched and vigils were held in Memphis and other major cities across the US, decrying the latest example of police brutality seen on video and calling for police reform.

The five charged officers were part of the department’s SCORPION unit, which was launched in 2021 to take on a rise in violent crime in Memphis. Shortly after the video footage of Nichols’ arrest was released, Memphis police announced the unit would be permanently deactivated as a sign the department was taking “proactive steps in the healing process for all impacted,” according to a January 28 statement.

Mulroy said his office has begun a review of prior cases involving the former SCORPION unit as well as the officers involved in Nichols’ death.

“We’ll also take a look at any other case that a defense attorney will bring to us if they have some … credible reason for thinking that the case might be suspect,” said Mulroy, adding that “manpower issues” are a concern in the “time-consuming process.”

Sixth officer fired, others face discipline

As the cases against the five officers move forward, fallout from the violent arrest continues for other members of law enforcement and first responders who were on the scene.

A sixth Memphis officer was fired but has not been charged, authorities said. Additional Memphis police officers may face further discipline, City Attorney Jennifer Sink told CNN last week.

Two Shelby County sheriff’s deputies who were at the scene of the arrest were found to have violated department policies and suspended for five days each without pay, according to a sheriff’s office statement obtained by CNN affiliate WHBQ.

Three Memphis Fire Department personnel were fired for their failure to promptly render emergency care to Nichols after the arrest, according to the fire department. In a letter to the Memphis City Council, Thomas Malone, president of the Memphis Fire Fighters Association, defended their actions, saying they “were not given adequate information upon dispatch or upon arrival on the scene.”

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