(CBS News) Investigators say they've found "suspicious circumstances around a possible entry point" at the home of missing six-year old Isabel Mercedes Celis. But Tucson police spokeswoman Sgt. Maria Hawke wouldn't comment on whether the entry point was a bedroom window or a door.
Family friend Mary Littlehorn said she heard from others close to the family that a window screen in the girl's bedroom had been knocked down.
The search for a missing first-grader in Tucson, Ariz., is now 48 hours old.
An entire block in the Tucson neighborhood has been blocked off. More than 150 law enforcement officers are systematically searching for the missing girl.
Hawke told reporters, "Officers are conducting a grid search. We have bloodhounds that are checking the area, as well. Picking up any scent of the little girl."
Her parents say she was last seen Friday night when they put her to bed, but when they went to wake her up on Saturday morning, she was gone. Her uncle, Justin Mastromarino, says the girl's parents are distraught. "Everything goes through your mind," he said. "You're angry. You're upset. You're frustrated. You're confused."
Early Monday morning, investigators and FBI urban search dogs scoured the Celis' family home for clues, finding evidence of a possible break in.
Sgt. Marco Borboa, of the Tucson Police Department, said, "The amount of leads that are coming in, we do have ample amount of investigators looking into those leads and they are all going to be looked into thoroughly."
The family fears Isabel may have been abducted in the same manner as Elizabeth Smart. Smart was 14 when she was snatched from her bedroom in the middle of the night. Smart was found alive nine months later. In 1993, 12-year-old Polly Klaas was abducted at knife point during a slumber party in her house. She was later found dead. In Tucson, the family has asked their church to start praying for Isabel's safe return.
Mastromarino said, "We need her home safe, safe and sound, you know, we like to hope that she just went for a walk and is gonna come back unharmed."
For Ben Tracy's full report, watch the video in the player above.
"CBS This Morning" senior correspondent John Miller, a former FBI assistant director, said this case is one of the unusual ones.
"Lots of children disappear from home. Those investigations usually find some family involvement," he said. "In this case, (and in the Elizabeth Smart and Polly Klaas cases), what you saw was sometimes it really does happen this way."
Bringing the FBI in, will help the investigation go forward quickly. He explained the FBI will bring in the C.A.R.D. team, the Child Abduction Rapid Deployment Task Force. "These teams are positioned at different places around the country. They can bring immediate resources within those first 24 to 48 hours. That's the critical time," he said. "Investigators, evidence response team technicians, FBI profilers who can look at various suspects and say, 'I would start with this one and move on to that one,' negotiators who, if there is a call for ransom, can coach the family through what to say and what not to say on those calls. So, that's what's going on behind the scenes in a case like that."
Miller added, "The other thing they'll do is, in the beginning of a case like this, particularly in a smaller area where a lot of leads are coming in very quickly, those leads can get lost. So, they bring in tools like Orion, a case management tool which computerizes every lead, puts it in a database and can match leads together when similar things come in more than once."
The Elizabeth Smart case, in particular, has influenced these types of investigations, Miller said. "One key thing that changed from Elizabeth Smart is the idea this kind of thing just doesn't happen, where somebody breaks into a home and steals a child and nobody hears it or sees it," he said. "I mean, it was an eye-opener for a lot of investigators (so now)...they can descend on (a case) with exactly the experts they need."
Watch his full interview in the player below.