What really happened on the day 6-year-old Etan Patz went missing in 1979 is still a mystery, as authorities have ended their search of a SoHo basement with no obvious finding of human remains, New York Police Deputy Commissioner Paul Browne said Monday.

No obvious human remains were found, but it's still a missing person case, Browne was quoted by the UK's Guardian as saying.

The suspected blood stain that was found in the basement has returned negative in a preliminary field test, a source familiar with the investigation told CNN. That stain as well as a hair will be analyzed at an FBI laboratory in Quantico, Va., the source told the network.

Etan disappeared on May 25, 1979, but wasn't officially declared dead until 2001. The missing boy's case was reopened in 2010 by Manhattan District Attorney Cyrus Vance Jr.

Investigators with the NYPD and the FBI spent four days excavating the basement of a building on Prince Street that used to be a workshop for local handyman Othneil Miller, looking for clothing and human remains that might yield evidence. The search was carried out after a cadaver dog detected a hit at the site.

Still, investigators have found no new leads.

Etan was seen at Miller's workshop the day before he went missing. Miller, now 75, maintains his innocence and has never been charged in the 33-year-old case.

Mr. Miller has been cooperating with this investigation for over 30 years, his attorney, Michael Farkas, told CNN on Friday. He has continued to cooperate on multiple occasions. And I am going to assist him in cooperating to the fullest extent possible.

CNN reported that an evidence team from the FBI, as well as the police, will conduct another check to ensure they didn't miss anything.

Miller was 42 years old when Etan vanished. The mysterious disappearance alarmed parents in New York and nationwide as the boy was walking two blocks to the school bus stop for the first time alone. Etan's face was the first to appear on the side of a milk carton.

Though the case remains unsolved, there has been speculation that a convicted pedophile by the name of Jose Ramos was involved. In 2001, a judge found Ramos responsible for Etan's death and ordered him to pay $2 million as a result of the civil lawsuit filed by the Patz family against him. Ramos, who never paid the money, knew Etan because he was familiar with the boy's babysitter. He is serving time in jail on an unrelated charge.