By Natasja Sheriff

NEW YORK (Reuters) - A jailhouse informant told the trial on Thursday of a man charged with murdering a New York boy in 1979 that another man, not the accused, bragged to him about committing the crime.

Pedro Hernandez, 54, a former grocery clerk, is on trial for kidnapping and murder in a case that hinges on his confession to police in 2012 that he choked 6-year-old Etan Patz, stuffed him in a box and left him in a New York alley.

Patz vanished as he walked alone for the first time to a school bus stop in his Manhattan neighborhood on May 25, 1979. His disappearance sparked a national movement to find missing children, with his picture one of the first to appear on milk cartons.

Defense attorneys say Hernandez is mentally ill and his confession was coerced by police.

They say the real killer is Jose Antonio Ramos, whose girlfriend used to walk Patz to school. For years, he was the prime suspect.

Testifying in state Supreme Court in Manhattan, informant Jeffrey Rothschild said he knew Ramos from prison. He said Ramos told him he had sexually abused Patz and boasted that the boy's body would never be found. 

Ramos told him the boy was dead but said there was no proof, he testified.

"Ramos admitted to me in the plainest possible language that he had sex with Etan," he said, calling Ramos an "explosively violent individual."

Patz has never been found. He was declared dead in 2001.

Hernandez was questioned and arrested in 2012 after police got a tip he confessed to the crime to a church prayer group in New Jersey.

Ramos, who is not charged in the case, served 27 years in a Pennsylvania prison for the rape and molestation of three young boys in the 1980s. He was released in 2012 but promptly rearrested for breaking sex offender registration laws and returned to prison.

Rothschild, who has a criminal record for credit card, mortgage and bank fraud, said he had cooperated on a number of cases with the federal Drug Enforcement Administration, New York and California prosecutors and the FBI.

Rothschild said he was shaken by Ramos' graphic description of he did to the boy.

"I thought my life was a total wreck," he testified. "I realized there were other people who were darker than me."

(Editing by Ellen Wulfhorst; Editing by Peter Cooney)