Updated

Eighteen inmates allegedly involved in the deadly riot at a Delaware maximum-security prison have been indicted on a series of charges, including murder.

The Delaware Department Justice charged 16 of the inmates with first-degree murder, assault and kidnapping stemming from the incident that started Feb. 1, 2017. Two others are charged with kidnapping, riot and conspiracy.

Inmates held four correctional workers hostage at James T. Vaughn Correctional Center in Smyrna during the riot, setting off a nearly 20-hour standoff.

Lt. Steven Floyd was killed and Correctional Officers Winslow Smith and Joshua Wilkinson were injured as a result of the riot and counselor Patricia May was kidnapped.

The siege ended on February 2 after two other guards were released, three others escaped and tactical teams broke through and rescued May.

The indictments were handed down Monday by a New Castle County grand jury. All of those charged are currently in Delaware prisons.

Eleven of the 18 inmates charged are already doing time for murder or manslaughter. Four of them are already serving life sentences.

"This was an extremely important and time-consuming investigation that involved unique challenges." Attorney General Matt Denn said in the statement. "I appreciate the police and prosecutors' focus on ensuring that justice is done for the victims in this case and their families."

The Associated Press contributed to this report.