(CNN) -- A 10-year-old Colorado girl reported missing one week ago is dead, police said Friday, spurring a manhunt for the person who killed her.
Westminster Police Chief Lee Birk told reporters that a body found earlier this week "has been positively identified" as that of Jessica Ridgeway.
"Our focus has changed from a search for Jessica to a mission of justice for Jessica," Birk said.
Jessica was last seen by her mother as she left for school October 5. Authorities then issued a statewide Amber Alert for the girl.
On Sunday, a man in a subdivision in Superior discovered Jessica's backpack outside his house about 7 miles from the girl's hometown of Westminster, said Westminster Police Investigator Trevor Materasso. Both towns are along the Denver-Boulder Turnpike.
A body -- which was "not intact," according to police -- was found Wednesday at a park in Arvada, a Denver suburb just five miles from Westminster. The body was identified Friday as that of Jessica.
With that news, Birk acknowledged that authorities "recognize that there is a predator at large in our community."
"We are doing everything within our power to apprehend that individual," he said.
Hundreds of volunteers had searched fields and streets, posted fliers and canvassed neighborhoods since news broke of the girl's disappearance. Hundreds of local, state and federal law enforcement officers have also been working the case, according to Birk.
As of Friday, authorities had searched more than 500 homes and 1,000 vehicles, responded to more than 1,500 tips and "covered thoroughly more than 800 leads," said Jim Yacone, special agent in charge of the FBI's Denver office.
"Open space and water searches by both law enforcement, the Westminster Fire Department and many hundreds of volunteers have been extensive, and they will continue," Yacone told reporters Friday.