By Edith Honan

NEW YORK (Reuters) - The fate of Etan Patz, the 6-year-old boy whose disappearance in 1979 helped galvanize interest in missing children, remained a mystery after a four-day excavation of a basement area near where Patz was last seen failed to turn up any new information.

Although the boy was formally declared dead in 2001, Manhattan District Attorney Cy Vance reopened the case in 2010 and investigators began tearing apart the basement last week looking for clothing and human remains after a cadaver-sniffing dog sensed something at the site.

But after four days of digging in Manhattan's SoHo neighborhood failed to turn up any new leads, the New York City Police Department planned to shut down and clean up the scene on Monday.

"No obvious human remains were found and it remains a missing person's case," said police spokesman Paul Browne.

One seemingly promising discovery - a stain found on a chunk of cinder block - was tested at the site, but came up negative for human blood, according to a law enforcement source.

Additional tests will be conducted in a laboratory, the source said.

On May 25, 1979, Patz's parents allowed the boy to make his first unaccompanied trip to the bus stop two blocks away. They never saw him again.

Patz was one of the first missing children in the United States to have his photograph printed on milk cartons, and his case helped fuel an intense national outreach campaign for missing children in the 1980s.

The revival of the search on Thursday by the FBI and New York Police Department raised hopes of a breakthrough but ended without finding any obvious human remains, said the source, who was informed on the matter but declined to be identified because of the sensitive nature of the case.

The investigation has long targeted Jose Antonio Ramos, a friend of Patz's babysitter who was later convicted of child molestation in a separate case. Ramos is serving a prison sentence in Pennsylvania and is due to be released in November.

In 2004 the Patz family won a $2 million civil judgment against Ramos, who has denied any involvement in the disappearance. The sum has not been paid.

The search of the basement, which had no known connection to Ramos, suggested law enforcement was pursuing a different theory. The space was used by Othniel Miller, a handyman, as a workshop at the time of Patz's disappearance, according to another law enforcement official who asked not to be named because the investigation was continuing.

The space was less than a block from Patz's home, an apartment in which Patz's parents still live.

Investigators recently interviewed Miller, who has not been charged with a crime. Miller's lawyer has said his client was not involved in the boy's disappearance and was cooperating with authorities.

(Reporting by Edith Honan; editing by Daniel Trotta and Mohammad Zargham)