The search for Etan Patz, a 6-year-old New York boy who disappeared more than three decades ago, is expected to resume Monday after it was suspended for "operational reasons," an FBI spokesman said.

A law enforcement source briefed on the investigation said no evidence of human remains has been found so far in the basement of a building in Manhattan's SoHo neighborhood where investigators are looking.

Around 2 p.m. Sunday, investigators searching the basement abruptly folded up a tent they had erected to shield them from a nasty rainstorm.

Moments later, two large New York Police Department vans rolled in, obstructing most of the view of the scene. Through a small break between the vehicles, photographers were able to catch a glimpse of something being loaded into the side of an unmarked blue van.

FBI spokesman Peter Donald declined to discuss the reasons behind the search's suspension. "We'll be back in the morning," he said.

Sunday's developments came a day after investigators discovered a possible blood stain on a concrete wall while tearing apart the basement in their search for clues in the case, a second law enforcement source told CNN.