NEW YORK â€"  Investigators searching for clues in the decades-old disappearance of New York 6-year-old Etan Patz are planning on resuming their search of a basement near his home Monday morning, an FBI spokesman tells myFOXny.

FBI spokesman Jim Margolin says the search of the basement was halted Sunday due to rain and other factors.

As the investigation continues in the basement, authorities are testing a suspicious stain found on the basement's wall last week, a source close to the case told FoxNews.com.

The source says when investigators sprayed Luminol on a wall in the basement, they detected "an organic substance" -- "possibly blood" -- and that the stain has been taken away for further testing. So far, about 50 percent of the basement has been removed.

The 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 said.

Investigators this week ripped up the basement's concrete floor with jackhammers and saws after a cadaver-sniffing dog picked up a scent in the 13-by-62-foot basement.

Law enforcement officials have said little about what they've uncovered so far in the basement. All potential evidence is being sent to FBI headquarters in Quantico, Va., for analysis. The source, who spoke to FoxNews.com on condition of anonymity due to an ongoing investigation, said the search of the basement could be completed as early as Sunday.  

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.

The boy's disappearance prompted an exhaustive search by police and nationwide media attention. Etan's photo was one of the first of a missing child to appear on a milk carton.

Investigators have long focused their attention on 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. Ramos is scheduled to be released from prison in November.

In 2002, Etan's father, Stan Patz, had Etan declared dead in order to sue Ramos in civil court. A civil judge in 2004 found Ramos 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.

Prosecutors, however, lacked enough evidence to charge Ramos criminally. 

In 2010, Manhattan District Attorney Cyrus R. Vance Jr. decided to reopen the case.

The focus of the investigation shifted to Miller after investigators received new leads in the case. Miller, originally from Jamaica, was interviewed after the boy disappeared and had what authorities deemed a solid alibi at the time. 

Authorities, however, noticed that Miller had a newly cemented concrete floor around the time of Etan's disappearance, but opted not to dig it up.

A source close to the case said "there is truth to that" when asked about news reports that Miller blurted out "What if the body was moved?" when taken by police to the basement in recent days. 

Miller has so far not been named a suspect and has denied any wrongdoing through his lawyer.

The Patz family, who has remained in the same apartment for 33 years in the hopes their son would one day return home, has declined to comment on the recent developments.

"To the hardworking and patient media people, the answer to all your questions at this time is no comment," reads a note outside their apartment, just one block away from dozens of FBI trucks and police cars. "Please stop ringing our bell and calling our phone for interviews."

"Stan Patz, 3E."

Click here for more on this story from myFOXny.com.

Fox News' Cristina Corbin and The Associated Press contributed to this report.