Updated at 8:01 p.m. ET: Child welfare officials have increased their involvement in the case of a missing 6-year-old Tucson, Ariz., girl and have forbidden the father from having contact with his other children, sources tell NBC News.

While Isabel Celis’ parents, Sergio and Rebecca Celis, have been cooperating with authorities since the child disappeared more than three weeks ago, neither parent has been ruled out as a suspect, police say.

“Until we find conclusive evidence that leads down one path, we are not going to close the doors on anything,” Tucson Police Chief Roberto Villasenor told NBC's "TODAY" on Monday.

Police say while involvement from Child Protective Services is common practice involving cases with missing children, “CPS instituted certain measures to ensure the continued welfare of the Celis children,” according to a statement released Friday.

In addition to Isabel, the Celises have two older sons, who are now in the custody of their mother, police say. The mother is no longer living with Sergio Celis, according to local media reports.

Isabel's father was reportedly alone when she disappeared and was the last one to see her missing. He reported her missing around 8 a.m. April 21 after her mother had left for work and he went to wake up the child. Isabel was not in her bedroom, he told police, and a room window was opened and its screen removed. Sergio Celis said he last saw Isabel around 11 o'clock the night before, when he put her to sleep.

Criminologist Casey Jordan, who has been following the Tucson investigation, said child welfare officials must have had established grounds for their recent involvement. “I don’t know that investigators are circling back to the family so much as they never left the family,” she told "TODAY."

Volunteers have canvassed the area, posting fliers that include a photo of the girl. Investigators have searched several homes in the neighborhood, dug up and combed the nearby landfill and searched drainage systems in Tucson.

More than 1,000 tips have poured into the police department, which has since reported spending $1 million in the investigation, according to local media reports.

Isabel's parents have made several public, emotional pleas for the safe return of their daughter in the days after her disappearance.

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Updated at 8 p.m. ET: Police have asked the family of a 6-year-old Arizona girl who disappeared from her bedroom to leave their Tucson home after an FBI dog search Monday turned up new information that required a closer examination, investigators told KVOA.com.

Police Chief Roberto Villasenor told the NBC affiliate that canine units alerted police to a "new direction" and officers were working with the dog handlers to interpret the latest discovery inside the home of missing first-grader Isabel Mercedes Celis. Dogs from an FBI canine unit from Virginia began searching the home after midnight Monday, police said. One dog has training in tracking scents, the other is a cadaver-sniffing dog.

"We have information obtained from the dogs that necessitate our follow-up investigation," Chief Villasenor said at a news conference. "In order to do that, we secured the residence. We've asked the family to leave the residence so we don't have to talk about any other contamination of the scene."

In an interview with TODAY, Villasenor said police were investigating more than 100 leads in the case. Police say Celis' parents last saw her in her room at 11 p.m. Friday and that she was discovered missing at 8 a.m. Saturday.

“I don’t want to talk about the details of the investigation, but we are not closing our mind to any possibility,” Villasenor told TODAY.

The police chief also confirmed that a window to the ground-floor bedroom was found open and that a screen from that window had been removed, but he stopped short of saying an intruder was believed to have entered the single-story house that way.

"That would be a potential point of entry that we've been interested in from the beginning," he said.

Isabel’s disappearance has shaken her Tucson neighborhood, and teams of volunteers have canvassed the area, posting fliers that include a photo of the girl. Isabel is described as 4-feet-tall with brown hair and hazel eyes. More than 200 people attended a Sunday evening vigil in an empty parking lot near the family home. Children pleaded for the little girl to come home, vowing they would do everything to find her.

More than 150 law enforcement officers have been involved in the search, police said. Villasenor told TODAY officers have searched the local landfill and transit station in efforts to find more clues, but there is no single lead on which they are focusing.

"If you start to do that too early and you start try and to focus on one particular path, you run the risk of losing other investigative leads or not recognizing those leads," Villasenor said.

Pacheco said investigators had questioned all 17 registered sex offenders in that area, a middle-class neighborhood consisting mainly of single-family dwellings, but those interviews turned up no clues.

He said officers had served at least two search warrants. The girl's parents, identified by friends as Becky and Sergio Celis, were helpful in the search for their youngest child, he said. Villasenor said police had classified the case as a "suspicious disappearance/possible abduction."

The family's minister, Miguel Mariano, parish priest of Saint Joseph Roman Catholic Church about a block away from Isabel's home, said he met with her parents and brothers on Sunday morning and prayed with them.

He described the family as looking distraught and distressed. "I asked, 'Do you need anything? Food? Anything from the community?' They said, 'No father, at this time we need your prayer,'" the priest recalled.

The Associated Press contributed to this report.

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