Attorneys for an inmate convicted in a prison guard's death are asking the U.S. Supreme Court to halt his Thursday evening execution.

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

If the execution is carried out Thursday, 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, Pruett's execution was rescheduled for October. Pruett's attorneys then unsuccessfully sought more DNA testing and filed a federal civil rights lawsuit in August, arguing Pruett had been denied due process. The 5th U.S. Circuit Court of Appeals rejected the lawsuit last week, and Pruett's attorneys appealed to the U.S. Supreme Court on Tuesday.