Condemned prisoner Michael Dewayne Johnson committed suicide early Thursday in his death row cell, slashing his throat and his arm just over 15 hours before he was scheduled to be executed, a prison official said.

Johnson used a makeshift blade fashioned from a small piece of metal attached to a wooden stick, said Michelle Lyons, spokeswoman for the Texas Department of Criminal Justice in Huntsville.

"He had used some sort of metal blade or razor to cut his right jugular vein and an artery inside his right elbow," she said.

Prison guards had been checking on Johnson's welfare every 15 minutes, as is customary, when they found him at 2:45 a.m. unresponsive in a pool of blood in his cell at the Texas Department of Criminal Justice Polunsky Unit.

"At a 2:30 a.m. check, Mr. Johnson was talking to prison staff, and had just eaten breakfast," Lyons said. "He had made no indications that he was contemplating suicide, nor has a note or other explanation been found."

He was taken to a hospital in nearby Livingston, where he was pronounced dead at 3:40 a.m., she said.

Johnson, 29, was scheduled for lethal injection shortly after 6 p.m. Thursday. He was convicted of the 1995 fatal shooting of Jeff Wetterman, 27, a Waco-area convenience store clerk. Johnson was 18 at the time of the crime.

Suicides on death row are not unprecedented in Texas, which has executed 377 prisoners since the state resumed carrying out capital punishment in 1982. Johnson's, however, was the closest to his scheduled execution.

Lyons said Johnson's death was being investigated by the department's Office of Inspector General. The source of the sharp metal piece was not immediately known.

An appeal to the U.S. Supreme Court to block Johnson's punishment was rejected by the justices last week, but his lawyer had asked for the court to reconsider its decision. A ruling on that matter was pending.

Johnson would have made the trip from death row to the death chamber in Huntsville, about 45 miles west, around midday Thursday.

In an interview with The Associated Press two weeks ago, Johnson said he wasn't the man who gunned down Wetterman, who helped pump gas at the family store south of Waco. Johnson blamed his companion, David Vest.

"I never even saw the dude," Johnson said. "(Vest) jumped back into the car and we took off. He hollered: 'Go! Go! Go!"'

Vest blamed the shooting on Johnson, took an eight-year prison term in a plea bargain and testified against his friend. Vest is now free.

Johnson was involved with other teenagers in what authorities said was a stolen car ring in Balch Springs, near Dallas, when he was arrested for the Wetterman slaying. At the time of the shooting, he was in a stolen Cadillac. The 9 mm pistol used in the shooting also was stolen.

Johnson and Vest were heading to Corpus Christi for a day at the beach to celebrate Vest's 17th birthday. With fuel low, and without cash, they pulled into the Lorena Fastime store on the Interstate 35 frontage road about 12 miles south of Waco. It was the practice at the store to help motorists with their fuel purchase.

"I guess Johnson was afraid if they drove off he'd get the license number and then police would be looking for them," said Crawford Long, an assistant district attorney in McLennan County. "They didn't have money to pay for the gas, and he just shot him in the head and killed him."

A friend testified at Johnson's trial that Johnson told him he shot Wetterman after Vest said, "Shoot!" Vest said he had uttered a similar-sounding expletive when he saw Wetterman come out to help them and knowing they didn't have the $24 to pay for the gasoline.

"I've never signed a statement, never signed any confession," Johnson said.

Vest, in his confession, admitted to the shooting. At Johnson's trial, he testified his companion was the shooter.

Vest's confession improperly was suppressed by prosecutors using "trickery and deceit" and knowlingly using false evidence to deprive Johnson of a fair trial, Johnson attorney Greg White said in his Supreme Court appeal.

"What he's trying to do is really ridiculous," Long said. "We indicted Vest as if he was the shooter. We indicted both of them that way. And Vest signed a stipulation of evidence that the indictment was correct.

"Now his defense attorney is trying to say Vest was admitting to the crime and being the shooter. It's simply not true."

Long also denied hiding the Vest confession, saying it was introduced to the trial judge and filed as part of the court record.

Johnson would have been the 22nd Texas inmate executed this year. Four more are scheduled to die over the next five weeks.