OLYMPIA, Wash. -- The father-in-law of missing Utah mother Susan Powell won't be released from prison Thursday, the Washington state Corrections Department said.

Steve Powell has been serving time on voyeurism charges for secretly recording images of young neighbor girls. He was sentenced in June 2012 to 2 1/2 years in prison.

May 23 was the earliest possible date Steve Powell could be released, based on sentence reductions he earned in prison for participating in programs and avoiding serious infractions, Corrections spokesman Chad Lewis said Wednesday.

To get out early, inmates must submit an acceptable release plan. Lewis said the state was still reviewing Powell's proposed plan. If it's approved, the state still requires 35 days to allow staff to notify victims and local law enforcement.

Police in Utah said this week they were closing the active part of the Susan Powell investigation and released their case file. She disappeared from her home in December 2009. Her husband, Josh Powell, was a focus of the investigation until he killed himself and the couple's two children last year in an explosive house fire in Graham, Wash.

Investigators turned their attention to Josh Powell's brother, Michael Powell, who, they now believe, helped Josh Powell dispose of Susan Powell's body. Michael Powell committed suicide by jumping off a parking garage in Minneapolis three months ago.

Josh Powell and his two young sons moved into his father's home in Puyallup, Wash., after Susan Powell's disappearance.

Steve Powell had a sexual obsession with Susan Powell and thoroughly documented it in journals seized by police. Officers investigating her disappearance searched his home.

Police in Utah said Monday they do not believe Steve Powell was directly involved with Susan Powell's disappearance but may know more about it than he has let on.

Police said both Steve Powell and Michael Powell were uncooperative in the investigation.

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  • Father Kills Sons And Self

    Pierce County sheriff's deputies and Graham firefighters work through the wreckage of a home in Graham, Wash. where the bodies of Josh Powell and his two sons were found on Feb. 5, 2012 in a double-homicide suicide. The boys had just been brought by a Child Protective Services worker to Powell's home for a supervised visit when police say the dad deliberately blew up the house. A judge had denied the previous week a request by Powell to regain custody of his sons, who had been living with the parents of his missing wife, Susan.

  • Pierce County sheriff's officials said it appears Josh Powell used an accelerant to increase the power of a gas fire that he intentionally set to kill himself and his two sons inside his Graham, Wash., home on Feb. 5, 2012. The saga of the Powell family began in 2009 when Josh Powell reported that his wife, Susan, had gone missing. That night he claimed he took his sons on an overnight camping trip in freezing temperatures.

  • Aerial View Of Josh Powell's Home

    The explosion at Josh Powell's home left smoldering remains in Graham, Wash. Police say Powell deliberately detonated the home moments after a Child Protective Services worker brought his sons Braden, 5, and Charles, 7, for a supervised visit.

  • Braden And Charlie Powell

    This undated photo provided by Chuck and Judy Cox shows their grandsons Braden, left, and Charlie Powell. (AP Photo/Courtesy Chuck and Judy Cox)

  • Chuck Cox, Braden Powell, Charlie Powell

    This undated photo provided by Chuck and Judy Cox shows them with their grandsons, Charlie, left, and Braden, right. Charlie and Braden were killed along with their father, Josh Powell, on Sunday in what police said was an intentional fire set by Powell. The Coxes are the parents of Powell's wife, Susan, who has been missing since 2009. (AP Photo/Courtesy Chuck and Judy Cox)

  • Braden And Charlie Powell

    This undated photo provided by Chuck and Judy Cox shows Braden, left, and Charlie Powell. (AP Photo/Courtesy Chuck and Judy Cox)

  • Police and other officials from Washington state and Utah view charred rubble, Monday, Feb. 6, 2012, at of the home where Josh Powell and his two sons were killed in the fire Powell set Sunday. (AP Photo/Ted S. Warren)

  • In this pool photo provided by the Pierce County Sheriff's Dept., a melted gas can is marked with an evidence flag, Monday, Feb. 6, 2012, in the rubble of the home in Graham, Wash., where Josh Powell and his two sons were killed. (AP Photo/Pierce County Sheriff's Dept., Ed Troyer, Pool)

  • Susan Cox Powell

    This image from<a href="https://www.facebook.com/pages/Friends-and-Family-of-Susan-Cox-Powell/158207040909324" target="_hplink"> Friends and Family of Susan Cox Powell</a> Facebook page shows missing Utah mother Susan Cox Powell. Powell has been missing since December 6, 2009. Her husband Josh Powell was a person of interest in her disappearance before he killed himself and their two sons on Feb. 5, 2012.

  • Chuck And Judy Cox

    Chuck and Judy Cox, the parents of missing Utah mother Susan Cox, talk to reporters at their home in Puyallup, Wash., Monday, Feb. 6, 2012. They had custody of their grandchildren, Charlie and Braden Powell, who were killed by their father Josh Powell during a supervised visit to his house on Sunday. Their daughter Susan Cox Powell went missing in December 2009 from the West Valley City, Utah home where she lived with her husband and sons. (AP Photo/Ted S. Warren)

  • Candlelight Vigil

    Supporters held an impromptu candlelight vigil for Braden and Charlie Powell, who were killed by their father Josh when he deliberately torched his Graham, Wash. home on Feb. 5, 2012. The brothers had been living with their maternal grandparents in a simmering custody dispute stemming from their mother's disappearance in Utah in 2009.

  • Flowers and two stuffed animals are left near crime-scene tape, Monday, Feb. 6, 2012, near the home where Josh Powell and his two sons were killed Sunday, in Graham, Wash., in what police said appeared to be a deliberately set fire. Powell's wife Susan went mysteriously missing from their West Valley City, Utah, home in December 2009. (AP Photo/Ted S. Warren)

  • Mourners posted signs and lit candles at Carson Elementary School in Puyallup, Wash. where Braden and Charles Powell attended school. The brothers were killed in an explosion caused by their father Josh Powell at his home on Feb. 5, 2012 -- days after a judge denied his request to regain custody of them.

  • A Pierce County fire investigator works his way through charred rubble, Monday, Feb. 6, 2012, at the home where Josh Powell and his two sons were killed Sunday, in Graham, Wash., in what police said appeared to be a deliberately set fire. Powell's wife Susan went mysteriously missing from their West Valley City, Utah, home in December 2009. (AP Photo/Ted S. Warren)

  • Josh And Braden Powell

    In this Aug. 19, 2011 photo, Josh Powell plays with his son Braden in a park near his home, in Puyallup, Wash. Powell had long been a person of interest in his wife. (Rick Egan | The Salt Lake Tribune Friday, August 19, 2011)

  • This photo from Facebook shows Susan Cox Powell in December of 2008.

  • A flier seeking information on the whereabouts of Susan Powell, who was reported missing Dec. 7, 2009, in Utah, is shown, Thursday, Dec. 17, 2009, at a press conference in Puyallup, Wash. Powell's family said Thursday they are saddened but not surprised that her husband Josh Powell has been named a person of interest in the investigation.

  • Josh Powell

    Josh Powell, the husband of Susan Powell, stands with supporters after a candle light vigil for his wife at the Church of Jesus Christ of Latter-day Saints in Puyallup, Wash., Sunday Dec. 20, 2009.

  • Judy Cox, the mother of Susan Powell, who was reported missing Dec. 7, 2009, in Utah, becomes emotional during a press conference, Thursday, Dec. 17, 2009, in Puyallup, Wash. Powell's family said Thursday they are saddened but not surprised that her husband Josh Powell has been named a person of interest in the investigation.

  • Charlie Powell

    This undated photo provided by Chuck and Judy Cox shows Charlie Powell, who was killed along with his brother Braden by their father Josh Powell in a double-murder suicide. (AP Photo/Courtesy Chuck and Judy Cox)

  • Braden Powell With HIs Grandmother Judy Cox

    In this undated photo provided by Chuck and Judy Cox, Judy Cox, right, sits at a table with her grandson Braden. (AP Photo/Courtesy Chuck and Judy Cox)

  • Braden Powell

    This undated photo provided by Chuck and Judy Cox shows Braden Powell. (AP Photo/Courtesy Chuck and Judy Cox)

  • Chuck Cox With Grandsons Charlie And Braden Powell

    In this undated photo provided by Chuck and Judy Cox, Chuck Cox sits with his grandsons, Charlie, left, and Braden, right, at the Coxes home in Pullayup, Wash. (AP Photo/Courtesy Chuck and Judy Cox)

  • Chuck Cox With Braden And Charlie Powell

    In this undated photo provided by Chuck and Judy Cox, Chuck Cox plays with his grandsons, Charlie, right, and Braden, left. (AP Photo/Courtesy Chuck and Judy Cox)