HUNTSVILLE (AP) — Attorneys for an inmate convicted in a prison guard’s death are asking the U.S. Supreme Court to halt his scheduled execution.

They want justices to review whether lower courts properly denied a federal civil rights lawsuit that sought additional DNA testing in Robert Pruett’s case and whether a prisoner who claims actual innocence, as Pruett does, can be put to death.

The execution is set for Thursday.

Pruett, 38, was already serving 99 years for a neighbor’s killing when he was convicted in the December 1999 death of corrections officer Daniel Nagle at a prison southeast of San Antonio. Nagle was stabbed repeatedly with a tape-wrapped metal rod and an autopsy showed the assault caused a heart attack that killed him.

Pruett would be the sixth prisoner executed this year in Texas, which carries out the death penalty more than any other state. Texas put seven inmates to death last year. His execution would be the 20th nationally, matching the U.S. total for all of 2016.

Pruett avoided execution in April 2015 when a state judge halted his punishment just hours before he could have been taken to the death chamber. His lawyers had convinced the judge that new DNA tests needed to be conducted on the steel rod used to stab the 37-year-old Nagle. The new tests showed no DNA on the tape but uncovered DNA on the rod from an unknown female who authorities said likely handled the shank during the appeals process after the original tests in 2002. In June, the execution was reset for the October date.

Pruett’s attorneys then unsuccessfully sought more DNA testing and filed the federal civil rights lawsuit in August, arguing Pruett had been denied due process. After the 5th U.S. Circuit Court of Appeals rejected the lawsuit last week, Pruett’s attorneys appealed to the U.S. Supreme Court on Tuesday.

In a second appeal to the Supreme Court, also filed Tuesday, the attorneys asked the high court to revisit the constitutionality of executing a prisoner who claims actual innocence in federal court because of newly discovered evidence after exhausting all other appeals. Supreme Court justices in 1993 ruled 6-3 in a Texas case that it was constitutional to do so.

State attorneys told the Supreme Court that Pruett’s appeals were intended to be a “further unjustifiable delay,” had been “repeatedly raised” and previously were “properly rejected” by the courts.

No physical evidence tied Pruett to Nagle’s death at the Texas Department of Criminal Justice’s McConnell Unit near Beeville. At his 2002 trial, prisoners testified that they saw Pruett attack Nagle or heard him talk about wanting to kill the guard. According to some of the testimony, he talked about possessing a weapon as well.

Pruett has said he was framed and that Nagle could have been killed by other inmates or corrupt officers at the McConnell Unit.

His trial lawyer, John Gilmore, recalled this week that Pruett had a horrible childhood environment and told of getting high with his parents when he was only 7 or 8.

Gilmore said he believed Pruett was going to be found not guilty of Nagle’s death. “But you’re defending somebody in prison already for murder. That’s already a big hurdle,” he said.

Pruett’s 99-year murder sentence was for participating with his father and a brother in the 1995 stabbing death of a 29-year-old neighbor, Raymond Yarbrough, at the man’s trailer home in Channelview, just east of Houston. Pruett was 15 at the time.

His father, Howard Pruett, is serving life in prison. His brother, Howard Pruett Jr., was sentenced to 40 years.

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