Courtney Sullivan, also a senior at Standley Lake, said her father spoke to her and her younger brother about Jessica's disappearance.
"He's definitely talked to us about being more careful about our surroundings. You could see why," said Sullivan, a cross-country runner who often uses neighborhood streets. "I'm running in places where there's lights, busy roads, where I can get to someplace if I need to."
Families in Arvada, near the park where the body was found, were already on edge over reports last month that a man tried to lure two young boys into a car in separate incidents. The cases remain unsolved.
"This person's around here," said Suzette Morgan, the mother of boys ages 13 and 8. "I would say that everybody around here is really freaked out."
Adults have been walking or driving children to school bus stops and talking with neighbors.
"We want to be the community that is visible, that is vigilant, that if somebody does come into our neighborhood or our community, that they can see that," resident Bob Ruet said.
On Thursday, authorities dropped their previous appeals to spread the word about Jessica's disappearance and instead asked for residents' help in identifying a possible suspect, signaling a shift in the focus of the investigation.
Retired FBI behavioral analyst Clinton Van Zandt told The Associated Press that tip-offs could include someone suddenly growing a beard, getting a new haircut or other changes in appearance. Other clues might be out-of-character behavior, such as someone detailing a car when he normally would have only washed it, Van Zandt said.
Police have said they don't suspect Jessica's parents, Sarah Ridgeway, who lives with Jessica in Westminster, and Jeremiah Bryant, who lives in Missouri.
The only substantive clue police have disclosed was the discovery of her backpack and water bottle in Superior, about six miles from her home. Police won't discuss what was found in the bag or test results involving it.
Associated Press reporter Thomas Peipert contributed to this story.