"She just boxed the little baby in and tried to light it," McCrary said. John's older brother, Michael, "came running in just before she got the flame to the flammable material."
McCrary said Adcock was later committed to a mental institution and is now deceased. She and Wexford Karr divorced in 1973, according to the Atlanta Journal Constitution.
Wexford Karr twice married and divorced women who were many years younger than him. He met Adcock, an 18-year-old traveling evangelist from Lovejoy, Ga., and married her when he was 37, according to McCrary.
The interview with John Karr's father and half brother came days after his first wife spoke to "Good Morning America." Quientana Ray was just 12 years old when she met Karr in 1984. Within a year, against her parents' wishes, she married Karr, who was 19 at the time.
Ray told "Good Morning America" that Karr frequently talked about young girls.
"He had fantasies about little girls he would talk to me about," she said.
Ray said her marriage to Karr was tumultuous. In 1985, after nine months of marriage, Ray filed for an annulment, complaining in court records that she was "fearful for her life and safety."
In a response filed with the court, Karr contested Ray's age, saying she was in fact 14. A judge later granted the annulment.
Karr later married a second time, to Lara Karr, who was 16 when their twin daughters died the day they were born on Sept. 1, 1989. The couple went on to have three boys in 1990, 1992 and 1993, and divorced in 2001.
Karr is expected to face first-degree murder and kidnapping charges for the Ramsey slaying.
In addition to first-degree murder, the counts against Karr in a sealed probable-cause arrest warrant include felony murder, first-degree kidnapping, second-degree kidnapping and sexual assault on a child.
Boulder, Colo., prosecutors have not revealed the evidence they have gathered against Karr, though critics have been skeptical about the truthfulness of his alleged confession because of inconsistencies between his statements and the circumstances surrounding JonBenet's slaying.
Karr's family has said that he was not in Boulder, Colo., at the time of JonBenet's killing. Lara Karr told ABC News affiliate KGO-TV in San Francisco that she was with her former husband in Alabama, where they lived at the time of JonBenet's killing, and that she did not believe he was involved in the slaying.
Karr, she said, was fascinated with the Ramsey investigation long before his arrest and spent a lot of time studying her case, as well as the case of Polly Klaas, the 12-year-old girl who was abducted from her Petaluma, Calif., home and slain in 1993.
Boulder County Sheriff Joe Pelle said Karr would be locked alone in an 8-by-10-foot cell, away from the facility's other 480 inmates. The county public defender's office has asked to meet with Karr, and Pelle said he expected a public defender will represent him.
The date of Karr's court appearance in Boulder may be announced today, according to the district attorney's office.