A suburban Dallas convenience store clerk was on the phone with his girlfriend when two people, one of them carrying a sawed-off shotgun, walked in. Gregory Martin told her he believed he was about to be robbed and to call police.

Plano officers found 15-year-old Christopher Vargas standing over Martin's lifeless body and 18-year-old Gustavo Garcia hiding in a beer cooler with the shotgun near him. Authorities later determined the weapon had been used a month earlier in a robbery at a Plano liquor store where the cashier, Craig Turski, was fatally shot.

Garcia, now 43, is set for lethal injection Tuesday night in Turski's 1990 slaying. He'd be the third prisoner executed this year in Texas, which puts more inmates to death than any other state.

A federal judge said Friday he won't stop the execution, and the Texas Board of Pardons and Paroles refused a clemency petition. No additional appeals are expected, Seth Kretzer, one of Garcia's lawyers, said Monday.

In the federal court appeal, Garcia's attorneys had argued that lawyers at his trial and in earlier appeals failed to uncover details of an abusive and alcohol- and drug-influenced youth — disclosures that could have convinced jurors to spare him from a death sentence. They also said they needed additional time to investigate those claims.

"Garcia's guilt is clear," responded Fredericka Sargent, an assistant Texas attorney general.

The U.S. Supreme Court last month refused to review an appeal that raised questions about deficient legal help, and last week turned down a request for a rehearing.

Court documents show Garcia, who has spent more than half of his life on death row, shot Turski in the abdomen on Dec. 9, 1990, then reloaded and shot the 43-year-old cashier in the back of the head. A month later, Martin, 18, was shot in the head after he was taken to a back room.

In a statement to police following his arrest for Martin's killing, Garcia said he'd ordered Turski to his knees when a customer entered the store.

"I then panicked," he said. "I shot the clerk with the shotgun."

On Thanksgiving in 1998, Garcia and five other inmates were scaling a pair of 10-foot-high prison fences when corrections officers opened fire on them and they surrendered. A seventh convict, Martin Gurule, was shot but managed to flee, making him the first inmate to escape Texas death row since a Bonnie and Clyde gang member broke out in 1934. Gurule's body was found about a week later in a creek a few miles from the prison, and an autopsy showed he drowned.

"At least I can say I tried," Garcia said of the escape attempt in a 1999 interview with The Associated Press. "Facing execution is scarier." He declined an interview request as his execution date neared.

Vargas, Garcia's partner in both fatal robberies, was tried and as an adult, convicted and is serving life in prison. His age made him ineligible for the death penalty.

At least nine other Texas inmates have execution dates set for the coming months, including three in March.