The Ronkonkoma grandmother of a baby boy who was killed four years ago told her neighbor she had a secret.

It was possible, Barbara Hertzler told her, that her daughter killed the baby instead of the man serving 20 years in prison for the crime, the neighbor testified Thursday at a hearing in Riverhead.

Carol Stalhut’s testimony about what Hertzler, her friend and neighbor, told her marked the start of a hearing to determine whether the first-degree manslaughter conviction of the baby’s father, Christopher Foster, 32, should be reversed. State Supreme Court Justice William Condon granted the hearing after Stalhut came forward in April.

During Foster’s trial last year, defense attorney David Besso suggested Barbara Hertzler, who suffers from paranoid schizophrenia and reported wanting to kill people, was the one who hurt 42-day-old Jonathan Hertzler. Now, however, his client’s hope for a new trial depends on Hertzler’s description of what she said her daughter did.

Stalhut said that description came on Feb. 11, during one of her daily visits to give Hertzler her medication.

“Sometimes she took too much,” Stalhut said during questioning by Besso.

advertisement | advertise on newsday

On this day, Stalhut said Hertzler was lucid. “She said to me, ‘Carol, can I tell you a secret?’” Stalhut said.

Hertzler then told her that the night Jonathan died, she saw her daughter, Clarissa, holding a blanket over the boy’s face and squeezing the sides of his head. Jonathan died of skull fractures. Stalhut said Hertzler told her that her daughter ordered her to go back to her room, so she left.

“Barbara was afraid of her,” said Stalhut, who struggled to maintain her composure during her testimony. “She was always afraid of Clarissa when she was on drugs.”

During Foster’s trial, Clarissa Hertzler, who was not charged, acknowledged years of addiction to OxyContin, followed by more years of taking the narcotic methadone to wean her from the pain pills.

Clarissa Hertzler testified during the trial that she initially stood by Foster, but eventually changed her mind in light of his opposition to having a baby with her and his carelessness handling the boy during his brief life.

Immediately after divulging her secret, Hertzler began to act erratically, Stalhut said. She walked to the window and made a Nazi salute, began picking up imaginary objects from the floor and filled garbage cans with water. Stalhut called 911 and her friend spent weeks in a psychiatric ward of a hospital.

Assistant District Attorney Laura Newcombe had objected to Stalhut testifying, arguing it was impermissible hearsay. Condon, however, allowed it. Hertzler, who did not testify at Foster’s trial, is expected to testify later in the hearing.

Besso said he believed this new evidence is enough to put the verdict in doubt. “I continue to believe in Chris’s innocence,” he said.