Authorities are examining "every facet" of missing 11-year-old Celina Cass' life, including computer records on the girl's home computer, the New Hampshire Assistant Attorney General tells FoxNews.com.
Celina, whom friends describe as sweet and reliable, was last seen by a family member Monday night at her home in Stewartstown, N.H., a small town near the Canadian border. She was reported missing Tuesday morning and those who know the child say it's unlikely she ran away.
Senior Assistant Attorney General Jane Young told FoxNews.com that the investigation is being treated as a missing person case.
"At this time, she is a missing child," Young said. "We're looking at all facets of her life."
FBI child abduction specialists as well as the New Hampshire and Vermont State Police and U.S. Border Patrol scoured fields, woods and the Connecticut River on Thursday for any sign of the girl, according to local reports.
Police have said that there's no indication Celina ran away or that someone took her, and there are no signs of a struggle.
Young said Thursday that authorities did not issue an Amber Alert because the case did not meet the criteria for one. Amber Alerts usually require a description of a vehicle or person the child was last with, Young said.
A flurry of activity was seen Thursday in and around the three-story house where Celina lives with her mother and stepfather. The New Hampshire Union Leader reports that state police detectives wrapped the driveway of the girl's home in yellow tape on Wednesday.
The FBI, meanwhile, said it had enlisted a four- to six-person "child abduction rapid deployment team" to pitch in. Young told FoxNews.com that the FBI had sent agents as far away as New York and Virginia to assist in the search. She also said law enforcement is canvassing "every house" in the neighborhood and "talking to everyone."
"She never went anywhere without her mom or sister," said family friend Rebecca Goodrum, 30, fighting back tears and holding a lit candle Wednesday night at a vigil held in a park in neighboring Canaan, Vt., about a half-mile from the girl's home.
"She is very shy," said Kayla Baglio, 18, who knows her, too. "If she doesn't know you, she'd look at her sister to see if it was OK to talk to you."
Celina's friend, 11-year-old Makayla Riendeau, described the girl as very athletic and a stickler about getting her school work done on time.
"She's a very good friend, and she never lets anybody down," Makayla said.
At midday Wednesday, about a mile north of town, five officers with the U.S. Fish and Game Department searched the woods behind an apartment building. They carried bags and boxes, but it was unclear if they collected anything.
Plainclothes police officers wearing purple rubber gloves were also seen surrounding a red pickup that was parked across the street from the girl's home, photographing it and looking inside. When news cameras began shooting pictures of what was happening, troopers moved a cruiser and a New Hampshire State Police SUV in front of the scene, to block the cameras.
The girl's disappearance hung heavy over Stewartstown, a community of 800 residents with one blinking streetlight and a handful of stores. Friends posted fliers of the girl on trees, utility poles, storefronts and car windows, and stood along the street in front of her house, waving motorists down to hand them copies.
"It's creepy," said Shannon Towle, who owns Towle's Mini-Mart on Route 3. "Things like this don't happen here. I know that's kind of a tired phrase. I'm an overprotective mom as it is. Now it's going to be way worse."
After sunset on Wednesday, about 80 people -- many with candles in hand and tears in their eyes -- gathered for the nighttime vigil. A framed picture of the girl sat on a picnic table, surrounded by candles. Friends, classmates and even people who didn't know Celina were among those in the crowd, and young children sobbed as adults comforted them.
Goodrum, of nearby Beecher Falls, Vt., said she was praying that Celina, whom she's known since she was 2, is safe.
"She was beautiful," said Goodrum. "She was the light of everything."
Towle said her 13-year-old daughter, Echo Towle, asked her mother whether she thought Celina was still alive.
"How do I answer that question? And do I want to?" Towle said. "I don't want to think about it, but I pray every second that she is.
FoxNews.com's Cristina Corbin and the Associated Press contributed to this report.