AUSTIN, Texas (AP) — An organization that supports death row defendants it says are wrongly accused argued Friday that newly discovered documents undermine the credibility of a key witness against a Texas man executed for the deaths of his three children based in part on arson evidence that has since been deemed faulty.

The case of Cameron Todd Willingham has been scrutinized by advocates who argue that Texas may have executed a wrongfully convicted man. Fire science experts already have repudiated much of the methodology used in his case.

The New York-based Innocence Project said Friday it has discovered a handwritten note that suggests a prosecutor gave a lesser charge to jailhouse informant Johnny Webb, who testified that told Webb he killed his daughters in 1991.

That would contradict claims made at trial by Webb and prosecutor John Jackson that Webb did not receive special treatment for his testimony.

"It's astonishing that 10 years after Todd Willingham was executed we are still uncovering evidence showing what a grave injustice this case represents," Barry Scheck, the Innocence Project's co-director, said in a statement.

A fire destroyed Willingham's home in 1991 and killed his three daughters. A state fire marshal who studied the scene testified at Willingham's 1992 trial that the fire was arson.

Scientists have since refuted many of the techniques used by arson investigators before 1992, including those used by the fire marshal in the Willingham fire. Attorneys submitted new scientific findings to Gov. Rick Perry in 2004 and asked for time to reopen the case, but Perry allowed Willingham's execution to go forward that year. Willingham maintained his innocence until his death.

The Texas Forensics Commission has determined the techniques used to convict Willingham were flawed, such as the belief that crackling of windows is an indication of arson. Scientists now know the crackling happens when firefighters spray cold water on them.

The Innocence Project has called for a posthumous pardon, but Perry has long declined to reconsider Willingham's guilt, calling him a "monster" who had killed his own children.

Lucy Nashed, a spokeswoman for Perry, said Friday that the governor's position has not changed.

"Todd Willingham was convicted and sentenced to death by a jury of his peers for murdering his three daughters, year-old twins and a two-year-old," Nashed said in an email. She added: "The governor agreed with the numerous state and federal courts that Willingham was guilty and the execution should proceed."

In a new filing Friday with the state pardon board, The Innocence Project said prosecutors worked to have charges for Webb, the jailhouse informant, reduced from an aggravated offense with a deadly weapon to a second-degree felony. Included in the filing is a handwritten note, found in files turned over by current prosecutors, that mentions a second-degree robbery offense "based on coop in Willingham." The note is not signed or dated.

Jackson, a former prosecutor who later became a state district judge, has maintained that Webb did not receive leniency. He told The Associated Press on Friday that he had not seen the note, but believed it was likely referring to efforts he made to get Webb out of prison after Willingham's trial due to threats on Webb's safety.

"The file may certainly reflect that we tried to get sentencing shortened, but it had nothing to do with any agreement relative to the Willingham trial," Jackson said.

Jackson said imprisoned members of the Aryan Brotherhood had threatened Webb due to his role in Willingham's conviction.

"We certainly had an interest in not seeing a primary witness in the case killed while he was in prison," he said.

Lowell Thompson, the current Navarro County district attorney, confirmed the note was in the files he allowed the Innocence Project to inspect, but did not take a position on whether it indicated anything about the case.

"I've seen it, but I'm not familiar with anything it might mean or it might not mean," Thompson said Friday.

Webb, who is being held in the Navarro County Jail on an unrelated aggravated assault case, could not be reached for comment.

The state fire marshal's office stood by their findings in the case until 2011, when the Texas Forensics Commission ruled the methods used were profoundly scientifically flawed.

The Texas Forensics Commission recommended a review of all arson convictions based on the same science used to convict Willingham. The recommendation came after the state attorney general limited the scope of its investigation into Willingham's case, prompting the panel to decline to issue a finding on any alleged negligence or misconduct by the State Fire Marshall's Office in the case.