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A Texas death row inmate who confessed to four slayings and at least nine rapes is set for lethal injection Wednesday amid concerns from his lawyers that his health issues make it likely his execution will cause him unconstitutional pain.

No one disputes Danny Paul Bible's guilt for a Houston woman's slaying nearly 40 years ago that went unsolved for two decades before a jury convicted him and sentenced him to death.

But instead of lethal injection, Bible's lawyers have proposed the 66-year-old be rolled in his wheelchair in front of a firing squad or be administered nitrogen gas to cut off oxygen to his brain until he stops breathing. His deteriorating health, the attorneys argue, has left his veins unsuitable for IVs to be inserted to carry a lethal dose of pentobarbital.

If either of those alternatives isn't possible — and state attorneys said neither is — Bible's lawyers said his punishment must be stopped.

"He has a number of medical problems getting progressively worse," Margaret Schmucker, one of Bible's attorneys, said Tuesday. "They're not going to be able to find a vein."

Bible's appeal went to the U.S. Supreme Court after the 5th U.S. Circuit Court of Appeals on Tuesday refused his civil rights lawsuit seeking a reprieve, a restraining order and an injunction. Assistant Texas Attorney General Stephen Hoffman said in a court filing that the lawsuit was a "meritless tactic" to delay his "well-deserved execution."

If the execution is carried out Wednesday evening, it would be the seventh this year in Texas, the country's most active death penalty state.

Lethal injection is the only execution method allowed in Texas and changing that would require approval of the state Legislature, which isn't scheduled to meet again until next year. A handful of death penalty states allow nitrogen hypoxia, although the method hasn't been used. Three Utah inmates have been executed by firing squad, the most recent in 2010. Utah now allows that method if drugs for execution are unavailable.

In Texas, Bible's lawyers argue that he has no suitable sites on his body for IVs and that severe tremors accompanying his Parkinson's disease would complicate insertion of needles. They also warn of a problematic execution like ones in recent years in Ohio and Alabama.

Hoffman, the state attorney, noted that IVs have been used recently to draw blood from Bible as part of his medical care.

Bible was a drifter with a record of violence in several states when he was arrested in Fort Myers, Florida, in 1999 for a rape in Louisiana. He told detectives in Louisiana about four Texas killings — including the death of a 4-month-old boy — and at least nine rapes, including five in San Jacinto County northeast of Houston.

The four slaying victims included 20-year-old Inez Deaton, a friend of Bible's cousin who was found on the banks of a Houston bayou in 1979. Investigators determined she had been raped and fatally stabbed with an ice pick. The killing went unsolved for decades before Bible linked himself to the case, and a jury decided in 2003 that he should die for the slaying.

The three other killings all occurred in North Texas on the same day in May 1983. The victims were Bible's sister-in-law, Tracy Powers; her 4-month-old son, Justin; and Powers' roommate, Pam Hudgins.

Bible pleaded guilty to Hudgins' death and was sentenced to 25 year in prison. He served seven and was released to Montana in 1992 on a form of parole known as mandatory supervision.

At his trial for Deaton's slaying in Texas, Harris County prosecutors presented evidence of robberies, thefts, assaults and abductions, including the rape of an 11-year-old girl in Montana and his confessions to repeated sexual assaults of young girls from 1996 to 1998.

"Some criminals' actions are so heinous, they earn the label 'worst of the worst,'" Harris County District Attorney Kim Ogg said.