OKLAHOMA CITY (AP) — Oklahoma Attorney General Gentner Drummond on Thursday appointed an independent counsel to conduct a comprehensive review of the murder conviction and death sentence of Richard Glossip, a death row inmate who has long maintained his innocence in a 1997 murder-for-hire killing.

Drummond announced the appointment of former District Attorney Rex Duncan to review all aspects of Glossip’s case, including the initial investigation by Oklahoma City police, two separate trials, sentencing and appeals.

“Circumstances surrounding this case necessitate a thorough review,” Drummond said in a statement. “While I am confident in our judicial system, that does not allow me to ignore evidence. This review helps ensure that justice is served, both to the (victim’s) family and the accused.”

Glossip, 59, has long maintained his innocence in the 1997 murder-for-hire killing of his boss, Oklahoma City motel owner Barry Van Treese.

A Houston law firm that looked into Glossip’s conviction last year reported lost or destroyed evidence in the case and that a detective who was biased against Glossip improperly asked leading questions to a co-defendant, Justin Sneed, to implicate Glossip in the killing.

“Our conclusion is that no reasonable juror, hearing the complete record, and the uncovered facts … would have convicted Richard Glossip of capital murder,” Stan Perry, an attorney for Houston-based Reed Smith who led the firm’s investigation, said after the report was released last year.

A telephone message left Thursday with Van Treese’s brother, Ken Van Treese, was not immediately returned.

Sneed, a handyman at the motel where Glossip was the manager, admitted to robbing Van Treese and beating him to death with a baseball bat. But he said he did so only after Glossip promised to pay him $10,000. Sneed was sentenced to life in prison and was a key witness against Glossip at both of his trials.

Glossip is scheduled to be executed on May 18.

He has been scheduled to be executed three separate times, only to be spared shortly before the sentence was set to be carried out. He was just hours from being executed in September 2015 when prison officials realized they had received the wrong lethal drug, a mix-up that helped prompt a nearly seven-year moratorium on the death penalty in Oklahoma.

Glossip’s attorney, Don Knight, said he was pleased with Drummond’s decision to seek a review of the case.

“The new evidence we have uncovered since 2015 shows conclusively, as the first independent investigation by Reed Smith found, that no reasonable juror who viewed all the evidence would find Mr. Glossip guilty of murder for hire,” Knight said in a statement. “We are confident that this new investigation will reach the same conclusion.”

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