Updated 10:49 PM ET
(CBS/AP) NEW YORK - FBI agents and New York City police officers were in a downtown New York City basement Thursday, searching for clues in the 1979 disappearance of Etan Patz.
CBS New York reports authorities have previously searched the building at Prince and Wooster streets where Patz lived.
Patz vanished without a trace in May 1979 after leaving his family's Manhattan apartment for a short walk to catch a school bus. It was the first time his parents had let him go off to school alone.
The building they searched on Thursday is about a block and a half away and on the same route Patz would have walked to the school bus.
Police spokesman Paul Browne said a forensic team was looking for blood, clothing or human remains. They are expected to be at the site for as many as five days. He wouldn't say what evidence led investigators to the property.
"The FBI and the NYPD are looking for evidence in the Etan Patz investigation. It's one of many leads that we're pursuing. It is a joint matter between the FBI and NYPD and no conclusions should be made about specifically why we're here or what we're looking for," FBI spokesman Tim Flannley.
According to CBS New York, a new search warrant was issued in the last week, allowing this search to begin Thursday.
FBI and police officials didn't publicly announce what led them to the site, but a law enforcement official told The Associated Press that investigators made the decision to dig after an FBI dog detected the scent of human remains at the building over the past few weeks.
Investigators have long eyed the basement with curiosity because it can be accessed from the street on the boy's route to school. At the time, the space was being used as a workshop by a neighborhood handyman who was thought to have been friendly with Etan.
FBI investigators have interviewed the man several times over the years. Investigators questioned him again recently, and as a result of those discussions decided to refocus their attention on the building, according to the law enforcement official.
The official spoke to the AP on condition of anonymity because the investigation is ongoing. Two other law enforcement officials also confirmed that an FBI dog had indicated the scent of human remains in the space.
District Attorney Cy Vance confirmed in May, 2010 that his office would reopen the case. "This was a shocking case at the time and it hasn't been resolved," Browne said.
Etan's disappearance drew national attention to child safety, ushered in a generation of parents who became afraid to send their kids out alone and helped fuel a movement to publicize missing children's cases. Etan's face was among the first to appear on milk cartons. President Ronald Reagan declared May 25, the day of his disappearance, National Missing Children's Day.
Etan's parents, Stanley and Julie Patz, became outspoken advocates for missing children. For years, they refused to change their phone number, in the hope that Etan was alive somewhere, and might call. They never moved, although they obtained a court order in 2001 declaring the boy dead.
Stanley Patz didn't respond to phone calls and email messages Thursday. A man who answered the buzzer at the family's apartment said they wouldn't be speaking to the media.
"We obviously are looking to bring closure to the investigation and to the family but that'll be determined throughout the course of this case," Flannley said. "We are obviously optimistic that we might find evidence but again people shouldn't draw conclusions from this. We're doing the best we can to bring closure in this matter."
No one has ever been prosecuted for the crime but the boy's parents sued an incarcerated drifter and admitted child-molester, Jose Ramos, who had been dating Etan's babysitter around the time he disappeared. However, Ramos denied killing the child, but in 2004 a Manhattan judge ruled him to be responsible for the death.
Ramos is scheduled to be released from prison in Pennsylvania this year, when he finishes serving a 20-year-sentence for abusing an 8-year-old boy.
Investigators have looked at a long list of possible suspects over the years, and have excavated in other places before without success.
The 13-foot by 62-foot basement space being searched Thursday sits beneath several clothing boutiques. Investigators began by removing drywall partitions so they could get to brick walls that were exposed back in 1979 when the boy disappeared, Browne said.
Browne said the excavation is part of a review of the case, which was reopened by the Manhattan district attorney two years ago.
"This was a shocking case at the time and it hasn't been resolved," Browne said.
The law enforcement activity forced the temporary closure of some businesses on the block, including the fashion boutique Wink, on the ground floor of the excavated building.
"It's insignificant," owner Stephen Werther said of the lost business. "It's retail. There's always another day for us to make a living. This may be the family's last chance to find out what happened to their son."