More than 100 law enforcement officials questioned sex offenders and scoured a northern Florida county today searching for a 7-year-old girl who vanished Monday while walking home from school. An Amber Alert has been issued for Somer Thompson, who went missing while on her way home from school with her twin brother, 10-year-old sister and friends in Orange Park, Fla. Officials said Somer walked ahead of her siblings after getting into an argument with some of the children, and was not home when they arrived. Sgt. Dan Mahla of the Clay County Sherriff's Office said authorities "suspect foul play" and said it would be "totally out of character" for the second-grader to run away and not come home. Somer was last seen just before 3 p.m. heading home from Grove Park Elementary School. She is 4-foot-5 and weighs 65 pounds. She was last seen wearing a ponytail with a red bow and carrying a purple Hannah Montana backpack and a lunchbox. At 4 p.m. the girl's mother called home and was told Somer had not arrived. The mother left work and contacted the sheriff's office. "This is an all-out search," Mahla said. "It's been go-time for us all night." The search included canine units, mounted police, dive teams and helicopters with heat-sensing technology. Several police agencies, the FBI and local volunteers began searching the wooded areas in near where Somer was last seen. Authorities have interviewed 57 registered sex offenders living in a three-mile radius of where the girl disappeared and plan to interview an additional 37 offenders in an expanded 5-mile radius. Mahla said in addition to the search operation, officers had followed up on 80 of the 150 leads people have called into the sheriff's office. However, he said, "there is no person of interest." He added that investigators have found "no physical evidence that might lead us to where she might be." Officials said they received reports nearly two weeks ago about passengers inside a blue Nissan Sentra trying to pick up another child near where Somer disappeared.