Somer's body was discovered in a landfill Wednesday about 50 miles away in Georgia after investigators followed garbage trucks from her neighborhood.
Justino told a news conference no witnesses have come forward to say they saw the first grader attacked or abducted.
Investigators have finished questioning all 161 registered sex offenders living within a 5-mile radius of Somer's home, Justino said.
"We feel that we do not have any suspects that are members of that group," she said.
An autopsy has been completed and investigators know how Somer died, but authorities won't disclose their findings or any details about the body, Justino said.
Somer's mother, Diena Thompson, appeared red-eyed on all three network television morning shows to say she believes investigators will catch the unknown killer.
"We're coming for you. We'll get you, and hopefully justice will be served," she said on ABC's "Good Morning America."
Missing child posters featuring Somer's face, with chubby cheeks and thick brown bangs, still plaster nearly every utility pole along the mile-long route from her elementary school to her home.
Authorities say Somer squabbled with another child Monday and then walked ahead of the group of kids and was never seen again. So far, the police have not made an arrest.
Somer's family was making funeral arrangements Friday. On Thursday evening, a steady flow of people - many of them parents, clutching the hands of young children - walked down that same road toward Somer's house to support her grieving family with a candlelight vigil.
Associated Press writers Brent Kallestad and Kate Brumback in Orange Park contributed to this story.