Authorities have found a suspicious stain on the wall of a basement being searched in the decades-old disappearance of 6-year-old Etan Patz, a source close to the case confirmed to FoxNews.com.

The unknown substance was detected Saturday in the former workspace of retired handyman Othniel Miller, 75, who was seen with Patz the night before he disappeared from Manhattan's SoHo neighborhood in 1979.

The source also confirmed reports that an unrelated molestation claim prompted police to reexamine Miller, who is now being described as a main "person of interest" in the Patz case. Miller's ex-wife told law enforcement last year that she divorced her husband in 1986 after learning he had sexually assaulted her 10-year-old niece a few years after Etan disappeared, the source confirmed.

Investigators this week ripped up the basement's concrete floor with jackhammers and saws, and were digging through the dirt in hope of finding the boy's remains after a cadaver-sniffing dog picked up a scent in the 13-by-62-foot basement.

Etan disappeared on May 25, 1979, while walking alone to his school bus stop for the first time, two blocks from his family's home. Miller's workshop was on the route the boy would have taken to his bus stop, authorities said.

There was an exhaustive search by the police and a crush of media attention in the days and weeks following Etan's disappearance. The boy's photo was one of the first of a missing child on a milk carton. Thousands of fliers were plastered around the city, buildings canvassed, hundreds of people interviewed. SoHo was not a neighborhood of swank boutiques and galleries as now, but of working-class New Yorkers rattled by the news.

A name gradually emerged as a possible suspect: Jose Ramos, a drifter and onetime boyfriend of Etan's baby sitter. In the early 1980s, he was arrested on theft charges, and had photos of other young, blond boys in his backpack. But there was no hard evidence linking Ramos to the crime.

Ramos, now 68, reportedly admitted trying to molest Etan on the day of his disappearance, but denied abducting him or killing him. Ramos has never been charged criminally in the Patz case and is currently serving a 20-year prison term in Pennsylvania for abusing an 8-year-old boy there.

A former federal prosecutor who had worked on the case declared in 1998 that he believed Ramos was behind Etan's disappearance and death.

Police investigated leads to Ramos at various points, including a 2000 search of the basement of the building where he lived in 1979. They dismantled the furnace and searched it for DNA. But they found only animal traces.

By the next year, Etan's father, Stan Patz, who never moved or even changed his phone number in the hope his son would reach out, had Etan declared dead in order to sue Ramos in civil court. He was tired of waiting for justice, he said at the time.

A civil judge in 2004 found him to be responsible for the disappearance and presumed death of the boy, after he disobeyed her orders to answer deposition questions under oath for a lawyer representing Etan's parents.

The ruling provided a tiny measure of comfort to the family. But the criminal case continues, and prosecutors lacked enough evidence to charge Ramos criminally.

The case was quiet until 2010 when new district attorney Cyrus R. Vance Jr. said he was going to revisit it.

Ramos is scheduled to be released from prison in November. His pending freedom is one of the factors that has given new urgency to the case.

But the focus of the investigation has shifted to the basement that had been used at the time as a workspace for Miller. He was interviewed after the boy disappeared. Investigators noticed at the time that the basement had a fresh concrete floor; his space was searched then but never dug up.

Miller gave investigators an alibi for the time of Etan's disappearance, though they are giving his account of the day a fresh look, a person familiar with the investigation said Saturday.

Law enforcement officials have spoken to him as recently as Wednesday, and one interview prompted them to take a closer look at the space, an official said, speaking on condition of anonymity because the case was ongoing.

Miller hasn't been named a suspect, and his lawyer said he has nothing to do with the case.

The 13-by-62-foot basement space being searched sits beneath several clothing boutiques. Investigators began by removing drywall partitions so they could get to brick walls that were exposed in 1979. The work will continue through the weekend. About 50 law enforcement agents, including forensics experts and an anthropologist, are on scene. While cadaver-sniffing dogs are capable of detecting scents much older than 33 years, it's also possible the dog picked up an animal scent or was plain wrong.

The cobblestone street remained closed off and was a veritable media circus, with trucks and crews parked along the curb and gawking tourists stopping to snap photos.

The Patz family hasn't commented or turned up near the site, though it's visible from their home - they've seen the circus before.

"To the hardworking and patient media people, the answer to all your questions at this time is no comment," read a handwritten note outside their door. "Please stop ringing our bell and calling our phone for interviews."
"Stan Patz, 3E."

FoxNews.com's Cristina Corbin and the Associated Press contributed to this report.