The number of law enforcement officers investigating the disappearance of an 11-year-old girl in this northern outpost grew Thursday to more than 100, as they turned a school into a bustling command post and went door to door seeking clues.

Celina Cass' home sat empty, surrounded by police tape and guarded by a state trooper. Her family stayed elsewhere during a third day of community-wide searches as townspeople continued to pass out fliers of the girl, who was last seen when she went to bed Monday night.

Investigators from state and federal agencies worked together in the Stewartstown Community School. Jane Young, a top investigator for the New Hampshire attorney general's office, said investigators were "not leaving any stone unturned."

"It is our greatest hope that we find her and bring her home safe to this community and her family," said Young, who described an "all-out search."

Police said there's no indication that Celina ran away or that someone took her, and there were no signs of a struggle. But the FBI brought in its child abduction rapid deployment team with agents from Virginia, New York and Philadelphia.

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More than 100 tips poured in, and investigators were acting on them.

A police motorcade traveled to the nearby home of Betty and Dustin Day, where Betty Day said investigators questioned her about a recent sleepover Celina had attended. From there, law enforcement officials continued farther down the road and were searching camps in the woods.

Outside a three-story apartment house where the Cass family lived, a state police major crime unit van arrived and technicians wearing booties entered the building.

The National Center for Missing and Exploited Children was asked to assist in mapping the locations of sex offenders in the area and sent a representative, retired police investigator Charles Masino, to Stewartstown to help investigators, said Ernie Allen, the center's president.

"This one is real scary," Allen said. "We're hopeful. There's a very aggressive effort under way to find her. So we certainly have not given up hope."

Investigators are likely working on several different theories simultaneously and ruling them out one by one, Allen said. Several areas of concern are the girl's young age, which makes it unlikely that she'd run away, the fact she was had been using a computer before she disappeared and the close proximity of the Canadian border, he said.

"The good news is that in these kinds of cases, which can be real scary, most kids come home safely," he said. "So just because it's been two or three days doesn't mean that there's no hope. There's an array of scenarios under which she could be out there."

Across the border, the Royal Canadian Mounted Police in the province of Quebec were alerted to the girl's disappearance, but there had been no sightings as of late Thursday afternoon, said Sgt. John Sparkes, an RCMP investigator in Sherbrooke, Quebec.

Authorities did not issue an Amber Alert for the girl because the case does not meet the state's criteria. New Hampshire issues its version of an Amber Alert only when authorities believe someone 17 or younger has been abducted, when they believe the child is in danger and when authorities have specific information they're asking the public to look for, said Jim Van Dongen, spokesman for the Division of Homeland Security and Emergency Management.

"They have to have some specific information they're asking the public to look out for, usually a description of a car, or a license plate number, or something tangible," said Van Dongen, who isn't involved in Celina's case. "It just doesn't say, `be on the lookout.' It's got to be something specific."

Celina's cousin Constance McKearney and another young woman, Cassie Belanger, spent Thursday morning gathering bottled water, juice and donations from local businesses and delivering them to the school for the investigators. They filled a minivan and carted the contents into the room where the investigators were gathered.

"We're just trying to say thank you for everything and we just hope she comes home safe," McKearney said.

The specter of the girl's disappearance has hung heavy over Stewartstown, a community of 800 residents with one blinking streetlight and a handful of stores.

Shannon Towle, who owns Towle's Mini-Mart on Route 3, said people want Celina home.

"It's really weighing on the town," Towle said. "The more time that goes by the harder it is. People just want her back."

Debbie Whelan, who dialed 911 after Celina's older sister Kayla went to her house looking for the missing girl Tuesday morning, said community residents continue to pray for her return even as unsettling thoughts enter their minds.

Whelan, who has six girls, several of whom go to school with Celina, said the 11-year-old never would've run away because she was too timid. Because of that, Whelan and many others believe someone went to the home to take her away.

"Is it someone from up here? Is it an Internet predator? We just don't know," said Whelan, who said people are now locking their doors.

In neighboring Canaan, Vt., scores of people gathered Wednesday night for a candlelight vigil for Celina.

"She was beautiful," said family friend Rebecca Goodrum, of nearby Beecher Falls, Vt. "She was the light of everything."

Click here for more on the disappearance of Celina Cass from MyFoxBoston.com.

NH police seek missing 11-year-old girl: MyFoxBOSTON.com