The FBI has joined the search for a missing 12-year-old Colorado girl who vanished on her way to a friend's birthday party.
The agency sent 15 detectives to Greeley, Colo., to look for Kayleah Wilson, Dave Joly of the Denver field office said.
"When it involves a child, we're there," Joly said, adding that the Greeley Police Department requested the extra help and expertise.
FBI agents and local police have spread out along possible routes Kayleah might have taken after she left home. At a brown two-story apartment building on 30th Street in Greeley, agents knocked on doors and handed out fliers with Kayleah's picture, hoping to drum up leads in the case.
"We're trying to leave no stone unturned, " FBI Special Agent Jonathan Grusing said. "Because with someone who is 12 years old, you don't know if they maybe ran into someone at these apartments and is staying there, thinking everything is OK."
Kayleah was reported missing by her mother, April Wilson, when she never came home Sunday evening from a birthday party that was supposed to end by 7 p.m.
Police, who have few leads, say Kayleah never made it to the birthday party.
"We do not believe Kayleah ran away. There is nothing to indicate she did," Sgt. Joe Tymkowych, a Greeley Police spokesman, said Wednesday. "We just haven't been able to come up with anything that indicates what may have happened."
Authorities are also reportedly questioning the 17-year-old boyfriend of Kayleah, of whom her mother told ABCNews.com she did not approve.
Kayleah had been seriously involved with the teen who was friends with her older brother, but the couple had broken up a few months ago, Wilson said.
The Greeley Tribune reported that Kayleah's ex-boyfriend is being questioned but is cooperating with police and has not been named a person of interest.
Wilson said that disappearing without calling or coming home is not typical of her daughter.
"She's never run away before," said Wilson. "I am worried she may be in danger because none of her friends have seen her, and it is like she just vanished, but that's not possible."
Kayleah, who is in sixth grade at Brentwood Middle School, was last seen wearing a white-and-pink shirt over a white tank top, blue jeans and white and red shoes. She is described as being 5 feet 1 inch, and approximately 145 pounds with brown eyes and brown hair.
Wilson said her daughter left at 3:40 p.m. Sunday for the party, which was just a few blocks away.
"She walked over to the couch and said, 'I love you,' and 'I'll be home by 7,' and she walked out the door and that was it," said Wilson. "She had been talking about the party. She was really looking forward to it."
Tymkowych said that since Kayleah was reported missing Sunday evening the search has taken over his police force.
"We have about 25 people out on the ground searches and recanvassing the areas where she was last seen," said Tymkowych. "We are reinterviewing a lot of the key figures in the investigation as well."
Tymkowych said that dogs had been used in the investigation initially but proved unsuccessful. He said that investigators working the case do not believe Kayleah had any problems with drugs or alcohol, and have not found anyone who may have wanted to harm her.
Kayleah Wilson Disappears From Her Colorado Neighborhood
"We have no evidence that there was any foul play," he said. "At this point, everything is still a question mark for us."
The eight-block area where Kayleah would have walked to get to the party is well-populated and safe, according to authorities, who are still looking alongisde a busy highway that runs behind her home.
Even so, Wilson said that she had no concern about her daughter walking alone and that the neighborhood is not dangerous.
"We all walk everywhere, we don't even have a car," said Wilson. "It's not a bad neighborhood."
Wilson describes her daughter as "loving" and "trusting," and said that she's always been well-behaved.
Other than that, Wilson said her daughter is a typical preteen who has a crush on one of the singers from the folk band Celtic Thunder.
On the morning of the day she disappeared, Wilson said her daughter had gone to church with her and had been happy during a lunch at a local restaurant with her grandmother.
"I just want everyone to help find her. I don't care if you're in this state or in another country, just help," said Wilson.
ABC News' Clayton Sandell contributed to this report.