STEWARTSTOWN, N.H. – Investigators went door to door Thursday in this northern outpost, seeking clues in the disappearance of an 11-year-old girl.

Canadian authorities were alerted to be on the lookout for Celina Cass while her home sat empty, surrounded by police tape and guarded by a state trooper. The family stayed elsewhere during a third day of searches.

Townspeople continued to pass out fliers of the girl, who was last seen when she went to bed Monday night. Dozens of investigators from state and federal agencies worked together in the Stewartstown Community School.

A police motorcade went to the 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 some camps in the woods.

Outside the Cass house, a state police major crime unit van arrived late in the afternoon, and technicians wearing booties entered.

Jane Young, a top investigator for the New Hampshire attorney general’s office, said authorities were “not leaving any stone unturned.”

“There’s still a very comprehensive search going on,” she said.

Police have 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 said it had brought in its four- to six-person child abduction rapid deployment team to pitch in.

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

“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 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.”

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.”

Local police decide whether those criteria have been met and, if so, call state police to issue the alert.

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.”

Scores of people gathered Wednesday night for a vigil for Celina in neighboring Canaan, Vt., where a framed picture of her sat on a picnic table, surrounded by candles.