Fifth-grader Jessica Ridgeway was last seen Friday walking to a park where she meets classmates before school in Westminster. A police flier with her photo was posted on the front door of her two-story home that was decorated for Halloween.
The FBI was assisting the investigation, and police stationed officers at schools in the area, though they said there's no indication other children are in danger.
"We have no reason to believe there is a person going around abducting children," Westminster police spokesman Trevor Materasso.
However, police in the nearby suburb of Arvada said they were investigating whether the disappearance of Jessica was related to two recent child enticement cases that involved a man who tried to lure two boys about the same age as Jessica into a car. No suspects were identified.
"Our detective is working to see if they're related," Arvada police spokeswoman Jill McGranahan said.
Jessica's mother lives in Westminster. Her father and great-grandmother Dana Moss live in Independence, Mo.
Moss told KSHB-TV that she consented to a search of her house because "we'd do anything in the world to get her back."
"Safely, alive, however we can have her back," Moss said.
In Westminster, law enforcement officers with dogs were searching Ridgeway's suburban Denver neighborhood, while about 100 others focused on the area 6 miles to the north where the backpack was found.
A resident in a neighborhood in Superior, a town along U.S. Highway 36, reported finding the backpack Sunday on a sidewalk. Police declined comment about its contents.
Materasso said it was sent to the state crime lab for analysis that involved DNA samples from the family.
People in the area have been moved to help in the search.
More than 800 volunteers turned out for Saturday's search and at least 300 leads have been called in to a tip line.
