Police spokesman Trevor Materasso said investigators were processing evidence from Pattridge Park and that no additional information was available. He left without answering questions.
In tweets, Westminster police said investigators had worked overnight to identify the body. Officers searched more of the park Thursday as well as areas closer to Jessica's home.
Police have ruled out her parents Sarah Ridgeway, who lives Colorado, and Jeremiah Bryant, who lives in Missouri. Authorities believe Jessica was kidnapped by an "unknown suspect."
Jessica's mother last saw her daughter walking to school. The girl never arrived, setting off a frantic search by hundreds of law enforcement officials and residents. Photos of the fifth-grader have been posted in the neighborhood and online since her disappearance.
Aurelio Florez, who has lived in Jessica's neighborhood for six years, said it was shocking that Jessica could have vanished during a two-block walk to a park where she usually met friends before continuing on to school.
"You can see the park from her front door," he said.
The only real clue police have revealed in Jessica's disappearance is the discovery over the weekend of a backpack and water bottle that she had with her when she disappeared. Police won't discuss what was found in the bag or test results on it.
The items were found in the town of Superior, some six miles from her home. The spot where the possessions were found is about 7 miles from the park.
Police initially said the public didn't need to fear a kidnapper, then that they were investigating whether the case might be related to that of another girl who was abducted for several hours Monday in Wyoming.
In that case, a man lured the girl into a sport utility vehicle, saying he needed help finding his puppy. The girl was discovered by hunters. The FBI said it does not believe the cases are related. Police are searching for a suspect.
Adding to the mystery was a reported sighting more than 2,000 miles away in Maine one of hundreds of leads being investigating from at least five states.
Westminster police repeatedly have urged the public to study the details of Jessica's face in a photo a small, gap-toothed grin, a slight bruise on her nose and a short home video, hoping someone may have seen something.
Additional police were sent to Jessica's school, said Lynn Setzer, spokeswoman for Jeffco Public Schools. The district has its own security officers at other middle schools and high schools.
Steve Saunders, a spokesman for nearby Adams County schools, said the district is trying to strike a balance between reassuring students and their parents that they are safe, while encouraging them to be vigilant.
Saunders said the district has security officers at all middle schools, but not elementary or high schools. He said the district will seek more help if authorities believe it is warranted.