TULSA, Okla. (AP) — An Oklahoma man already serving time for murder has pleaded guilty to using contraband cellphones to run a large-scale meth ring from his maximum-security prison cell.

The Tulsa World reports that 35-year-old Slint Tate's Thursday plea agreement with federal prosecutors calls for serving 20 years after his state sentence. Tate likely won't serve more time because he's currently serving life without parole.

Court records show Tate admitted to conspiring with six others to help sell methamphetamine throughout Oklahoma in 2016. Tate's common-law wife Robin Zumwalt also pleaded guilty Thursday to using a cellphone to talk to Tate about coordinating the drug ring. She faces up to four years in prison.

Tate's sentencing is scheduled Sept. 11. He's been at the Oklahoma State Penitentiary since 1999 for a deputy's shooting death.

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Information from: Tulsa World, http://www.tulsaworld.com