HOUSTON — 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.

Oklahoma prison authorities have blamed a collapsed vein, not the drugs, for the injection problem, though an investigation is ongoing.

Campbell’s lawyers argue that the problem is the secrecy surrounding the drugs, and that the drug supplier’s name is necessary to obtain and test the efficiency of the drugs – arguments that have been rejected by federal courts in Texas and other states in recent months .

“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.”

Texas’ 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 execution drug by citing possible threats of violence to the supplier if it’s disclosed. Texas uses a single drug, pentobarbital, while Oklahoma used a three-drug combination.

Campbell’s attorneys said it doesn’t matter that different drugs were used in Oklahoma, because all of the potent drugs have potentially serious side effects. The risk to inmates like Campbell, who was convicted of abducting, raping and killing a woman in 1991, “derives primarily from the secrecy of the entire process – not the individual drug that might be used in one execution vs. the next,” the lawsuit argues.

Questions about execution procedures have drawn renewed attention from defense attorneys and death penalty opponents as states have been forced to scramble to find new sources of execution drugs. Several drugmakers, including many based in Europe, have refused to sell drugs for use in executions.

The issue surfaced in Texas – the nation’s most active death penalty state – when it replenished its execution drug stock in late March. But the U.S. Supreme Court rejected appeals that focused on secrecy surrounding the drug supplier’s name, and three Texas inmates have been executed since then.