NEWARK -- As jury selection began in preparation for his murder trial, an Essex County jail inmate on Wednesday admitted to killing his cellmate, pleading guilty to a manslaughter charge under an agreement that will see him spend an additional two decades in prison.

The Prosecutor's Office had charged Rahdi Richardson with murder after authorities said his cellmate, Desmond Sanders, was found beaten and strangled on Sept. 1, 2013 at the Essex County Correctional Facility.

Richardson ultimately pleaded guilty Wednesday to aggravated manslaughter, after jury selection had already begun in his trial before Judge James W. Donohue, according to Katherine Carter, a spokesperson for the Essex County Prosecutor's Office.

Richardson agreed to a sentence of 29 years in prison, Carter said in an email.

At the time of Saunders' killing, Richardson was being held in connection with a June, 2013 armed robbery in Newark, to which he also pleaded guilty Wednesday, according to Carter.

The state's case against Richardson is being prosecuted by assistant prosecutors Justin Edwab and Joseph Giordano.

Richardson's sentencing has been scheduled for March 27.

Thomas Moriarty may be reached at tmoriarty@njadvancemedia.com. Follow him on Twitter at @ThomasDMoriarty. Find NJ.com on Facebook.

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