In the 10 years since Texas executed Cameron Todd Willingham after convicting him on charges of setting his house on fire and murdering his three young daughters, family members and death penalty opponents have argued that he was innocent. Now newly discovered evidence suggests that the prosecutor in the case may have concealed a deal with a jailhouse informant whose testimony was a key part of the execution decision.

The battle to clear Mr. Willingham’s name has symbolic value because it may offer evidence that an innocent man was executed, something opponents of the death penalty believe happens more than occasionally. By contrast, Justice Antonin Scalia wrote seven years ago that he was unaware of “a single case — not one — in which it is clear that a person was executed for a crime he did not commit.”

Mr. Willingham was convicted on charges of setting the 1991 fire in Corsicana, Tex., that killed his three children, and was sentenced to death the next year. The conviction rested on two pillars of evidence: analysis by arson investigators, and the testimony of a jailhouse informant, Johnny Webb, who said that Mr. Willingham had confessed the crime to him.

The arson investigation has since been discredited; serious questions were raised about the quality of the scientific analysis and testimony, which did not measure up to the standard of science even at the time. But the prosecutor who led the case shortly before Mr. Willingham’s execution argued that even though the arson analysis had been questioned, the testimony of Mr. Webb should be enough to deny any attempt for clemency.