“No,” he said. “Mommy and Eric was there.”

The next day, she called the lead sheriff’s investigator to tell him what the boy had said and that she no longer suspected that her son-in-law, Michael Morton, had killed her daughter, Christine. She urged the investigator to abandon the “domestic thing now and look for the monster.”

Days after Mrs. Morton’s badly beaten body was found in her bed in August 1986, someone used her credit card in another city. And a check was cashed with her forged signature.

The sheriff’s investigators who saw Mr. Morton as the prime suspect had that information and a transcript of the grandmother’s call. But when he was on trial facing a life sentence for murder, his defense lawyers knew none of it.

A quarter-century later, after six years of fighting for DNA tests that now almost certainly will result in the reversal of Mr. Morton’s conviction, his lawyers say prosecutors withheld this and other exculpatory evidence from his original defense lawyers and from the trial judge despite orders to turn it over. In court filings, the prosecutors have denied accusations of wrongdoing.

Since 1994, DNA tests have exonerated 44 Texas inmates, according to the Innocence Project of Texas, based in Lubbock. In the wake of those cases, Texas lawmakers have made significant reforms to criminal justice procedures to help prevent wrongful convictions. But defense lawyers and Mr. Morton’s advocates argue that under antiquated Texas discovery laws, the alleged injustices that robbed him of 25 years could still happen.