A former inmate at the juvenile detention center in Casper has agreed to a plea deal that would require him to serve two to four years in prison, a prosecutor said Monday.

Carlos Arellano, 17, is expected to plead no contest to third-degree sexual assault as part of the agreement, according to Assistant Natrona County District Attorney Michael Schafer.

Arellano's attorney, Patrick LeBrun, declined comment. A plea agreement must be formally accepted by a judge during a court hearing.

Prosecutors in April charged Arellano with sexually assaulting two younger cellmates at the Regional Juvenile Detention Center a month earlier. The charges followed a Natrona County Sheriff's Office investigation.

Arellano's trial was set to begin Monday.

Another former inmate, Harlan Buddy Taylor, 18, had already pleaded no contest to aiding in one of the alleged assaults. As part of the deal, prosecutors will seek no more than a five-year prison sentence for him.

Arellano and Taylor are being prosecuted as adults. Both were removed from the juvenile detention center after the allegations arose.

Arellano is charged with sexually assaulting a 15-year-old inmate while Taylor held the younger boy inside a dormitory-style cell they were all housed in. Arellano is also accused of sexually assaulting a 16-year-old boy in the same cell on a different occasion.

The 16-year-old did not implicate Taylor in that alleged assault.

Arellano pleaded not guilty to the charges at his June arraignment. He faces the possibility of life behind bars if convicted on all counts.

The center houses up to 40 juvenile offenders, ages 12 to 17, in the Hall of Justice in downtown Casper. At the time of the incidents, it was being operated by Cheyenne-based Frontier Correctional Systems.

Since the allegations surfaced, Cornerstone Programs Corp. of Colorado has taken over operations from Frontier.

A separate Wyoming Department of Family Services investigation found that an inmate at the center had been sexually assaulted and that staff members and supervisors there failed to report it properly, according to documents released in June.

Officials haven't confirmed whether the state and local investigations are related.

Reach crime reporter Joshua Wolfson at (307) 266-0582 or at josh.wolfson@trib.com.