Early in the week, investigators said they had no evidence suggesting foul play.
But Deputy Chief Jim Kerns changed course Friday, saying new evidence recovered in a Thursday night search of the apartment of the boy's mother, Melissa Scott Jenkins, has detectives tracking new leads.
"The evidence we've uncovered shows that there are suspicious circumstances surrounding Robert's disappearance," Kerns said during a press conference. "Volunteers assisting in the search today are being given the information by search team officers that Robert may indeed be injured or the victim of a tragic event."
So far, police say there are no arrests or suspects in the case.
Jenkins has been at previous press conferences, but didn't attend Friday. Police wouldn't say where she was.
The boy was visiting her the night he disappeared. His father, Charles Manwill, has had custody since 2008 and lives in New Plymouth, about 45 miles northwest of Boise.
According to court records, Jenkins pleaded guilty in March to a misdemeanor charge of injury to a child following an October 2008 incident that fractured her infant son's skull. Jenkins "did willfully inflict" the injury to her other son "by striking the child's head on a surface, causing a fracture to the child's skull," on Oct. 19, 2008. She was sentenced to 29 days of work release, fined $75.50 and put on probation for two years, according to court documents.
That child is the son of Jenkins' boyfriend, Daniel Edward Ehrlick. The boy was in the care of the state Department of Health and Welfare through at least February, according to court documents.
Investigators were seen taking an SUV and other items during the search of Jenkins' apartment Thursday night.
Boise police spokeswoman Lynn Hightower said more than 1,000 volunteers showed up early Friday morning to help search, making it the biggest missing-person search in the city's history.
In the early days of the search, police were aided by the help of hundreds of volunteers who scanned neighborhoods surrounding the mother's apartment complex.
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