Kent Sprouse acknowledged almost immediately after he was arrested more than a decade ago that he killed a police officer and another man outside a Dallas-area convenience store. A jury decided in 2004 that he should be put to death, and unsuccessful appeals since then have focused on whether Sprouse was mentally ill at the time of the slayings in 2002 and should be spared the death penalty. No new appeals are pending as Sprouse's lethal injection scheduled for Thursday evening approaches. He would be the fifth inmate executed this year in Texas, the nation's most active death penalty state. Death row: Kent Sprouse gunned down Ferris Police Officer Marty Steinfeldt, 28, at a gas station outside of Dallas before also shooting dead a customer, Pedro Moreno, 38, in 2002 . Sprouse was convicted of the death of Ferris Police Officer Harry Marvin 'Marty' Steinfeldt III, who was shot outside the Ferris Food Mart about 20 miles south of Dallas. Steinfeldt, 28, was responding to a 911 call about a customer shot outside the store. Witnesses said Sprouse carried a shotgun into the store the afternoon of Oct. 6, 2002, while he made a purchase and then walked outside and fired toward two men at a pay phone. He went to his car and appeared to have some trouble with it, then shot and killed a customer, Pedro Moreno, 38, who was pumping gas nearby. As Steinfeldt drove up, he was met with gunfire and hit twice under the arm where his vest did not protect him. He managed to fire 17 shots and wound Sprouse in the chest, leg and hand. Court records indicate that Sprouse told an officer who accompanied him to a hospital that he believed Moreno was an undercover officer, so he shot him. 'And I shot the other officer that was in uniform,' Sprouse said, according to the records. Medical tests showed Sprouse, a Boone County, Missouri, native, had taken illegal drugs within the previous 48 hours. 'He started hitting the meth and went crazy,' Jim Jenkins, his lead trial lawyer in 2004, recalled last week. 'Even his family was afraid of him.' Kent Sprouse, 42, was sent to death row for a 2002 shootout at a Ferris, Texas, convenience store that killed police Officer Harry Steinfeldt III (right) andcustomer  Pedro Moreno, whom Sprouse had asked for help repairing his car . Jurors rejected an insanity defense and pleas for a life prison term. Sprouse was charged but not tried for Moreno's death. 'It was a senseless taking of two human lives,' Don Maxfield, the now retired Ellis County chief felony prosecutor who handled Sprouse's trial, said last week. 'It's just a sad story all the way around.' The U.S. Supreme Court in November refused to review Sprouse's case. Ferris Police Chief Sam Love said he and several of his officers plan to be outside the Huntsville Unit prison Thursday evening to show support for their deceased colleague. Steinfeldt had been with the Ferris department about two years after serving as a police officer in the Army. 'I don't think it's too soon or too long or whatever,' Love said of Sprouse's scheduled lethal injection. 'My feeling is the criminal justice system has done its work and we can move on now.' Scene: Sprouse is expected to be executed about 6 pm Thursday in Huntsville, more than 150 miles southeast of Dallas, by lethal injection. The process is expected to take 15 to 30 minutes at most . Texas Department of Criminal Justice officials said a recent purchase of pentobarbital means they have enough of the sedative to carry out Sprouse's execution and three others set for this month. The state's supply will need to be replenished for at least three other executions that are scheduled in May and June. Death penalty states have found it increasingly difficult to acquire drugs as manufacturers, bowing to pressure from death penalty opponents, refuse to sell to state prison agencies for use in capital punishment.
Kent Sprouse, 42, faces lethal injection at 6pm Thursday .
In 2002 he gunned down Ferris Police Officer Marty Steinfeldt, 28, at a gas station outside of Dallas and a customer, Pedro Moreno, 38 .
Sprouse was high on meth, but his insanity defense was rejected .
He was sentenced to death in 2004 .
Sprouse has not appealed the sentence recently .
It could be the last execution in Texas for a while .
The state has a shortage the lethal drug pentobarbital .