Authorities have intensified efforts to locate Celina Cass, an 11-year old New Hampshire girl missing since Monday, though no Amber Alert has been issued.

Residents of Stewartstown, N.H.  -- an 800 person town a mile from the Canadian border -- have banded together to assist a frantic search for the schoolgirl, who is described as "shy" and unlikely to have run away. 

The FBI and the Royal Canadian Mounted Police have joined the search effort, and on Friday the Connecticut River was lowered to aid in a helicopter search for evidence of Celina's whereabouts.

Fox News reported that police have cordoned off the home that Celina Cass shares with her mother, sister and stepfather. Cass' family and the other occupants of the converted apartment building have been relocated while investigators search phone and computer records. The home is being guarded by a state trooper.

ABC News obtained court records showing that Celina's stepfather, Wednell Noys, has an arrest record for violating a restraining order and threatening his ex-girlfriend. In 2003, he was forcibly committed to a hospital with a diagnosis of schizophrenia.

It was Noys who first discovered Celina missing, when he entered her bedroom on Tuesday morning.

The Boston Herald reported that police officers were seen on Wednesday taking photographs of the interior of a red pickup truck parked across the street from Celina's home. Officers moved official vehicles in front of the pickup truck to obstruct media photographers' attempts to capture images.

Celina's mother and stepfather have not made a statement since their daughter disappeared.

Fox News reported that an Amber Alert has not been issued because the circumstances do not meet the state's criteria. "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 Jim Van Dongen, spokesman for the Division of Homeland Security and Emergency Management, who is not directly involved in the case.

"It just doesn't say, 'be on the lookout.' It's got to be something specific."

Officials told ABC News that the disappearance is still being treated as a missing person case.

"Every house, every individual in the neighborhood is being spoken to," New Hampshire Assistant Attorney General Jane Young said Thursday.

Those who knew Cassie believe the girl would not have run away.

"She never went anywhere without her mom or sister,"  family friend Rebecca Goodrum is quoted in the Boston Herald as saying during a candlelight vigil Wednesday night.

"She is very shy," added Kayla Baglio, 18, who also knew the girl.  "If she doesn't know you, she'd look at her sister to see if it was OK to talk to you."

Celina is 5-foot-5 and weighs 95 pounds, with long brown hair and hazel eyes and was last seen wearing a pink shirt, a pink pullover, blue shorts and shoes.

Any information about Celina Cass' whereabouts should be directed to New Hampshire State Police at 603-846-3333.