Isabel Mercedes Celis Missing: Search continues in Tucson, Arizona for 6-year-old girl

(CBS/AP) TUCSON, Ariz. - Tucson police continue to search for 6-year-old Isabel Mercedes Celis, who was reported missing from her bedroom Saturday morning.

Pictures: 6-year-old vanishes from Arizona home

Investigators early Monday sent FBI dogs trained for urban searches into the home while law officers worked to determine if Isabel was abducted.

Police say they found a "possible entry point" at the home, but did not elaborate further. According to a family friend, a window screen was knocked down. The disappearance of the first-grader has prompted a massive search of the area.

Two days of searching by scores of police and officers failed to locate Isabel, who police said was last seen by her family in her bedroom at 11 p.m. Friday. When she was discovered missing at 8 a.m. Saturday, they called authorities.

The dogs arrived from the FBI's Virginia headquarters late Sunday and began searching at the home around midnight, said police Sgt. Marco Borboa.

"We have deployed the dogs and they're working at the residence," he said Monday.

Officers kept the whole neighborhood block where Isabel lives cordoned-off for a second day Sunday.

More than 150 law enforcement officers were involved in the effort, which included a fourth search of a three-mile radius around the home in temperatures that reached the high-90s, police Lt. Fabian Pacheco said at a Sunday evening news conference.

He said the search was being scaled back during the overnight hours.

Investigators found "suspicious circumstances around a possible entry point" at the home police, Sgt. Maria Hawke said. But she 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.

Earlier Sunday, Tucson police chief Roberto Villasenor said officers had served at least two search warrants. The girl's parents, identified by friends as Becky and Sergio Celis, were helpful as police worked to find their youngest child, he said. He said police were still classifying the case as a "suspicious disappearance/possible abduction."

"We're not ruling anything out of the investigation at this point because we really need to keep our mind open about all the information that's been brought to us," Villasenor said. "The family has been cooperating with us."

Investigators were looking into various scenarios, including the possibility that Isabel wandered out of the home she shares with her parents and two brothers. Hawke said Sunday the wandering off theory was becoming less likely as time passed.

In addition to the highly trained dogs, authorities said they have also started the process of checking on the whereabouts of sex offenders in the area.

Volunteers have been posting fliers of the girl - who is described as about 4-feet-tall with brown hair and hazel eyes - in gas stations, malls and fast food restaurants.

More than 200 people attended a Sunday evening vigil in an empty parking lot near the family home.