The search underway today is at 127 Prince St., in the city's SoHo neighborhood. The basement was the workshop of Othneil Miller in 1979.
Federal sources told ABC News that the night before the boy disappeared he had been in the basement where the handyman befriended Patz and gave him a dollar.
Just prior to today's search a law enforcement cadaver dog got a hit in the basement that could indicate the current or past presence of human remains.
According to sources, the area of the basement where the dog picked up the scent appears to be one that had been resurfaced with fresh concrete at or shortly after the time of Patz's disappearance.
Sources told ABC News that even if a body had been kept for 24 hours or less and then moved, a trained dog could pick up the scent decades later.
The basement was searched in 1979, the year the boy disappeared, but the floor was never dug up.
The floor will also be dug up in a search for human remains, clothing or other evidence.
The material removed will be taken to another site and preserved. Current forensics will allow authorities to look into hollows and to perform sophisticated DNA analysis on any potential evidence.
The search for Patz has been one of the largest, longest lasting and most heart wrenching hunts for a missing child in the country's recent history.
"This process right here, this process that you're witnessing, will take upward of five days," Browne said.
Patz vanished on May 25, 1979 in the SoHo neighborhood of Manhattan while walking alone to a school bus stop for the first time.
22 years later: A judge declares Etan Patz dead in June of 2001 at the request of his parents who want to pursue a wrongful death lawsuit against Jose Antonio Ramos, the man they believe killed their child. Ramos is currently in jail for child molestation and has allegedly confessed to other inmates that he killed Patz.