The decision came after earlier trials of seven inmates resulted in only one - who admitted planning the uprising knowing it could become violent - convicted of murder in guard Steven Floyd's death at James T. Vaughn Correctional Center.
Prosecutors have opted to dismiss cases against six of the 18 inmates originally charged in the riot and will move forward only against defendants Roman Shankaras, Lawrence Michaels and Alejandro Rodgriguez-Ortiz. Prosecutors have previously described Shankaras as the "mastermind" behind the February 2017 riot, while other inmates have testified that Michaels, nicknamed "Smoke," participated in attacking Floyd.
"Prosecutors have an obligation only to prosecute criminal cases where they believe there is a reasonable likelihood of a conviction at trial based on the evidence," the attorney general's office said in a statement Friday. "Prosecutors in the Vaughn trials - who are among the Department's most experienced and who have done a remarkable job in an exceedingly difficult case - have evaluated the evidence against the remaining defendants in light of the testimony in the first two trials and the results of those trials."
The prosecution of the inmates has already cost Delaware taxpayers more than $1.1 million in defense attorney costs.
With little physical evidence, and no surveillance camera footage, prosecutors have relied heavily on testimony from other inmates who were in the prison building during the riot but were not among the 18 charged. Defense attorneys have consistently pointed out that the prosecution witnesses have contradicted one another, and that their testimony has conflicted with statements they gave investigators.
The first trial of three inmates resulted in Dwayne Staats being convicted of felony first-degree murder, first-degree murder of a law enforcement officer, two counts of assault, four counts of kidnapping and second-degree conspiracy. He was already serving a life sentence for murder.
Co-defendant Jarreau Ayers was acquitted of murder but convicted on the same additional charges Staats faced. The jury acquitted a third co-defendant, Deric Forney, on all charges.
The second trial ended last month with no convictions for any of the four inmate defendants.
Floyd was killed during the 20-hour uprising. Two other officers were beaten and tormented by inmates before being released. Response teams eventually used a backhoe to breach a wall and rescue a female counselor. She was not injured.
One of the initial defendants, Royal Downs, a former Baltimore gang leader serving a life sentence for murder, pleaded guilty to a single count of riot and agreed to testify for the prosecution against his fellow inmates. Another defendant, Kelly Gibbs, killed himself in November, just days after pleading guilty to rioting, kidnapping and conspiracy.
Shankaras, meanwhile, recently completed a 7-year sentence for unrelated riot and robbery charges but is being detained on $2.8 million cash bail on the prison riot charges.
Michaels is serving 19 years for kidnapping, attempted robbery and other crimes, with a projected release date of May 2024.
Rodriguez-Ortiz is serving 40 years for manslaughter and other charges and is scheduled to be released in November 2044.
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