By Natasja Sheriff

NEW YORK (Reuters) - Jurors finished a 17th day of marathon deliberations on Thursday without reaching a verdict in the murder trial of a man who confessed to strangling Etan Patz more than three decades ago.

The 6-year-old boy disappeared in 1979 as he walked alone for the first time to his school bus stop in Manhattan's Soho neighborhood.

The case shook New York City and changed the way the United States responds to reports of missing children. Patz's picture was one of the first on milk cartons in a nationwide campaign to locate abducted or missing children.

Pedro Hernandez, 54, a former deli worker, is on trial on kidnap and murder charges in state Supreme Court in Manhattan. During its lengthy deliberations, the jury has twice told the judge it was deadlocked but has been ordered to keep trying to reach a verdict. [ID:nL1N0XW1W8]

The panel was set to return to court on Friday for an 18th day of deliberations.

Hernandez in 2012 told police that he choked the boy in the basement of a deli where he worked, stuffed him in a box and left his still-moving body in an alley.

Hernandez's attorneys argued he is mentally ill and that police coerced his confession. They say the real culprit is a longtime suspect, Jose Ramos, whose girlfriend walked Patz home from school. Ramos, convicted of sexually abusing boys, is serving a prison term in Pennsylvania.

Prosecutors say Hernandez is a cunning criminal whose confession was voluntary.

Patz was never found and was declared dead in 2001.

The deliberations are believed to be among the longest in New York state criminal court history, said David Bookstaver, a spokesman for the state Office of Court Administration. The agency does not keep records on the length of jury deliberations.

Bookstaver said the only deliberations that come close in the past three decades were are the 12 days it took jurors in 1987 to find three white men guilty of manslaughter in the racially charged Howard Beach killing.

In that case, a small group of black men walking into predominantly white Howard Beach, a neighborhood in New York City's Queens borough, were confronted, then chased by a group of white men. One of the black men ran onto a busy highway and was struck and killed by a car.

(Editing by Barbara Goldberg and Jonathan Oatis)