First-grader Isabel Mercedes Celis's parents last saw her in bed at 11 p.m. Friday, and they discovered her missing at about 8 a.m. Saturday, Tucson police spokeswoman Sgt. Maria Hawke said.
After a fruitless day of searching Saturday, numerous patrol and search and rescue personnel continued the hunt Sunday morning, Hawke said.
She said that at any given time, 75 to 100 officers from a variety of agencies were taking part, including police, FBI agents and a large contingent of deputy U.S. Marshals.
Police officials couldn't be reached early Sunday, but the Pima County Sheriff's office said that as of 2:30 a.m. there was still no word that the girl had been located.
Teams combed an area of Tucson Saturday around East Broadway Boulevard and Craycroft Road into the evening using street patrols, canines, detectives and a helicopter.
At one point late Saturday night, communications operator Patrick Olea said the area encompassed "pretty much the entire east side."
Also involved are Department of Corrections bloodhounds, search and rescue experts and a Pima County rapid response team.
Friends of the family also fanned out to distribute fliers with a photo of Isabel, KVOA-TV in Tucson reported.
"We're really surprised or shocked that anything like this could happen to our family," the girl's uncle, Justin Mastromarino, told the television station.
Hawke said earlier that investigators were looking into all potential scenarios, including the possibility that Isabel got up and wandered out of the home she shares with her parents and two brothers.
But late Saturday, when asked if police still think she might have wandered away, Hawke responded: "We are treating it as a suspicious disappearance and possible abduction."
Investigators also examined every door and window of the house for signs of a break-in, Hawke said.
Both parents live in the home, so police had no indication a child custody dispute was involved but weren't completely ruling it out.
"All potential avenues of investigation have been vigorously explored throughout the day and evening," Hawke said.
Neighbors told KVOA-TV that they couldn't understand how something like this could happen to the Celis family.
"They're amazing people. They're very nice very sweet (and) outgoing," said neighbor Jerry Pike.
The working-class neighborhood of single-family homes is sandwiched between a large shopping mall to the east and businesses and a Catholic school to the west.
Isabel is described as just under 4-feet-tall and weighing 44 pounds, with brown hair and hazel eyes.
Associated Press
