In this November 15, 2012 file photo, Pedro Hernandez, right, appears in Manhattan criminal court with his attorney Harvey Fishbein, in New York.

New York -

A man charged with an infamous killing of a six-year-old New York City boy in 1979 pleaded not guilty on Wednesday to murder, even though police say he confessed in the sensational case that spawned a nationwide movement to publicise cases of missing children. Lawyers for the defendant say the admission is false.

Pedro Hernandez, 51, wore a gray sweatsuit and answered “not guilty” at the hearing in the case of Etan Patz. Hernandez's lawyer insisted his confession to police was false.

“My client had no motive and no history,” defence attorney Harvey Fishbein said outside court.

“There is a serious question as to what happened in May 1979,” Fishbein said. “There is no crime scene. There are no witnesses to a crime.”

Fishbein also noted a body was never recovered.

He said that Hernandez confessed falsely after seven hours of questioning by police, and that he is still under medical and psychiatric care.

“My client is not happy that he is in jail,” Fishbein said.

Fishbein said he would seek to dismiss the case because the only evidence is his client's false confession. Under state law, a confession can be enough to convict someone as long as authorities can establish a crime occurred and the confession is reliable.

But if the case does go forward, Fishbein said the defence will revolve around his client's mental state, though he isn't pursuing an insanity defence and Hernandez was found mentally fit to stand trial.

An insanity defence would mean acknowledging he committed the crime but arguing that he was too psychologically ill to know it was wrong. Hernandez didn't kill Etan and made a false confession because of his mental problems, among other factors, Fishbein said.

“The only part that mental disease plays in this case is its role in the confession,” he said before the court date.

Psychiatric exams of the jailed Hernandez have found that he has an IQ in the borderline-to-mild mental retardation range, his lawyer has said. Hernandez also has been found to suffer from schizotypal personality disorder, which is characterised by hallucinations, according to his lawyer.

The defendant's wife and daughter attended the hearing but did not speak to reporters.

Etan's disappearance led to an intensive search and garnered huge publicity. His photo was among the first put on milk cartons, and his case turned May 25 into National Missing Children's Day.

Hernandez was a teenage stock clerk at a convenience store when Etan disappeared on his way to school on May 25, 1979. Hernandez was a married father with no criminal record and living in Maple Shade, New Jersey, when police approached him based on a tip this year. The tip came after federal authorities and police dug up a basement in the neighbourhood hoping for clues, putting the cold case back into the limelight once again.

Investigators say Hernandez told them he lured the boy into the convenience store with the promise of a soda. According to police, he said he led the child to the basement, choked him and left his body in a bag of trash about a block away.

Etan's parents, Stan and Julie Patz, have been reluctant to move or even change their phone number in case their son tried to reach out. They have not commented.

Etan was declared legally dead by his father more than a decade ago so he could sue convicted child molester Jose Ramos in the boy's death. Ramos was found responsible - a ruling made because he didn't entirely cooperate with questioning during the lawsuit - and Fishbein could seek to make that a factor in Hernandez's defence.

Ramos, now 69, had been dating the boy's baby sitter in 1979 and was the prime suspect for years, but he was never charged. He repeatedly denies having anything to do with Etan's disappearance.

Ramos was later convicted of molesting two different children. He completed a 27-year sentence last month but was immediately arrested upon his release from a Pennsylvania prison, because authorities said he had given them a false address for where he'd be living.

Ramos is currently jailed in Pennsylvania on charges of failing to properly register as a sex offender under Megan's Law immediately after he was released earlier this year from prison. - Sapa-AP