On May 20, 1999, the day before his younger son’s high school graduation, John Thompson was scheduled to be executed in the Louisiana State Penitentiary. He had survived six other execution dates, but he was running out of miracles.

Early in 1985, when he was 22, Mr. Thompson had been arrested in New Orleans for a carjacking and for the separate murder of a local hotel executive.

By 1999, he had spent 14 years on death row at the notorious maximum-security prison farm known as Angola, named for the nation from which blacks who worked on a former plantation there had been abducted by slave traders.

Just 30 days before his scheduled execution, a private investigator hired by his lawyers stumbled upon a forgotten microfiche.