By Natasja Sheriff

NEW YORK (Reuters) - Jury deliberations resumed for a 13th day on Friday in the trial of a former deli worker who confessed to the 1979 killing of Etan Patz, a New York City boy whose picture was among the first to appear on milk cartons in a U.S. campaign to locate missing children.

The panel has been struggling to decide kidnapping and murder charges against Pedro Hernandez, 54, in the death of 6-year-old Patz. After telling Justice Maxwell Wiley at state Supreme Court in Manhattan that it was deadlocked on Wednesday, the jury has been returning to court on his orders to continue trying to reach a verdict.

In 2012, Hernandez confessed to police that he had choked Patz in the basement of the deli, stuffed him in a box and left the boy's still-moving body in an alley.

But Hernandez's attorneys argued that he is mentally ill and that police coerced his confession.

Patz's 1979 disappearance from his Soho neighborhood in Manhattan brought national attention to the issue of abducted children and changed the way the United States responds to reports of missing kids.

Patz vanished on May 25, 1979 as he walked alone for the first time to a school bus stop. The crime long haunted New Yorkers who can recall the massive search for the missing blond boy, who was never found. He was declared dead in 2001.

Hernandez's defense attorneys have put the blame for the boy's disappearance on Jose Ramos, whose girlfriend walked Patz home from school and who was long considered the prime suspect.

Ramos, convicted of sexually abusing boys, is serving a prison term in Pennsylvania.

Hernandez's wife, Rosemary, and daughter, Becky, have been present in court throughout the jury's deliberations.

Patz's father, Stan Patz, who was present throughout the trial, has been absent from court since the jury began deliberations.

If the jury stalemate holds and a mistrial is declared, the prosecution will have to decide whether to retry the case.

(Editing by Barbara Goldberg and Jonathan Oatis)