Texas prison officials must reveal the source of their lethal injection drugs to avoid an incident "as horrific" as last week's bungled execution in Oklahoma, attorneys for a Texas death row inmate argued Tuesday in an attempt to delay his upcoming execution.

Lawyers for Robert Campbell filed a federal civil rights lawsuit citing the Oklahoma execution that went awry when an intravenous line of lethal drugs became dislodged, a failure that went unnoticed for 21 minutes. The inmate, Clayton Lockett, later died of an apparent heart attack.

Campbell's lawyers say it doesn't matter that Texas uses a single drug, pentobarbital, while Oklahoma used a three-drug combination because all of the potent drugs have potentially serious side effects. They also use arguments that have been rejected in recent months by federal courts in Texas and other states: that the problem is the secrecy surrounding the drug, and that the drug supplier's name is necessary to obtain and test the efficiency of the drugs.

"The possible cause of Mr. Lockett's botched execution are all issues that have been, are, or could be problematic in Texas," according to the lawsuit filed in U.S. District Court in Houston. "There is a substantial risk that Mr. Campbell's execution could be as horrific as Mr. Lockett's."

The state Department of Criminal Justice, which is among the named defendants in the lawsuit, "does not comment on pending litigation," agency spokesman Jason Clark said Tuesday. Like several other states, Texas has refused to identify the source of its pentobarbital by citing possible threats of violence to the supplier if it's disclosed.

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