"Justice will not be denied" for Robert Manwill, said Boise Deputy Chief Jim Kerns in a short 6 p.m. press conference Wednesday.
The Ada County Coroner's autopsy showed that the 8-year-old boy's death was "not an accident," he said.
Police "will determine how and when Robert died, and who is responsible," Kerns said.
He said that no evidence exists that Robert was abducted by a stranger.
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Ada County Coroner Erwin Sonnenberg said Wednesday that dental records confirmed that the boy found Monday in the New York Canal was the 8-year-old.
Robert was reported missing at 10:11 p.m. July 24 from an Vista Avenue-area apartment complex.
Kerns said the criminal investigation would proceed "with urgency," but he said the police would "compromise the legal procedures" and the integrity of their case by prematurely releasing details.