(Adds conclusion of search at scene)
By Michelle Nichols and Jonathan Allen
NEW YORK, April 22 (Reuters) - Investigators searching a New York basement for clues about the 1979 disappearance of a 6-year-old boy have ended their digging at the scene after "nothing conclusive was found" from a four-day excavation, a law enforcement source said on Sunday.
Etan Patz was one of the first missing children in the United States to have his photograph printed on milk cartons after he disappeared, and his case helped fuel an intense national campaign in search of 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 bones or 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 search in the Patz location has concluded and there was nothing conclusive found," the source said. "A stain on the wall is being tested (in a laboratory) but it's inconclusive what that's about. The digging has been done. There were no bones or obvious human remains found."
However, an FBI spokesman insisted a team would be back at the site in the trendy Manhattan neighborhood of SoHo on Monday.
"The search is going to continue, and we're not done, and we'll be back out there tomorrow," said Peter Donald, an FBI spokesman.
The stain, described as a spot on a wall, was found on Saturday in a basement that was used by 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 boy was formally declared dead in 2001. Investigators recently re-opened the case and began tearing apart the basement looking for clothing and human remains after a cadaver-sniffing dog sensed something at the site.
Investigators found the stain by spraying the site with luminol, a chemical that can help indicate the presence of even trace amounts of blood, among other substances.
In 1979, Othniel Miller, then a handyman, used the basement as his workshop, 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 he is cooperating with authorities. A woman who answered the door at Miller's red-brick terraced home in Brooklyn on Saturday deferred all questions to Miller's lawyer.
"This is so overwhelming," she said, before heading back upstairs. "We just want peace."
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. President Ronald Reagan would later declare that date to be National Missing Children's Day.
No one was criminally charged in the disappearance, but in 2004 the Patz family won a $2 million civil judgment against Jose Antonio Ramos, a friend of Patz's babysitter who has denied any involvement in the disappearance. The sum has not been paid.
Ramos was convicted of child molestation in a separate case and is serving a prison sentence in Pennsylvania. His sentence is due to expire in November. (Additional reporting by Edith Honan; Editing by Daniel Trotta and Paul Simao)