Updated

Attorneys for a Texas death row inmate have filed a federal civil rights lawsuit seeking to delay his execution following a bungled execution in Oklahoma.

Lawyers for Robert Campbell said Tuesday that Texas prison officials must reveal the source of the pentobarbital to be used in Campbell's execution scheduled for May 13. Otherwise his punishment could be "as horrific as" Oklahoma's attempt to execute Clayton Lockett last week.

Lockett's execution went awry when an intravenous line of lethal drugs became dislodged — a failure that was not noticed for 21 minutes despite the man's evident discomfort. Lockett died of an apparent heart attack.

Campbell's lawyers say it doesn't matter that Texas uses a different execution drug.

Campbell was convicted in the 1991 rape and slaying of a Houston woman.