The effort to find fifth-grader Jessica Ridgeway also focused on the area around the home in Westminster where she lives with her mother, and a neighborhood 6 miles to the north where the backpack believed to belong to her was found Sunday.
Ridgeway was last seen Friday walking to a park where she meets classmates before school. 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 investigators have been in contact with relatives in Independence, Mo.
Jessica's great-grandmother Dana 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," she said.
The Associated Press couldn't immediately reach Moss.
In Westminster, police stationed officers at elementary schools to assist children, though they said there was no indication that other children were in danger.
"We have no reason to believe there is a person going around abducting children," police spokesman Trevor Materasso said.
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 in those incidents, which took place about 6 miles south of Jessica's home.
"Our detective is working to see if they're related," Arvada police spokeswoman Jill McGranahan said.
A resident in a neighborhood in Superior, a town along U.S. Highway 36 linking Boulder and Denver, reported finding the backpack 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.
Authorities got a late start searching for Jessica because her mother works nights and slept through a call Friday from school officials after Jessica didn't arrive for classes.
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.
So many police fliers have been posted on fences and windshields around the city that authorities were having trouble keeping up with demand for the material.