CHICAGO (CBS) — Saying the last two people known to have seen 2-year-old Kyrian Knox alive are not cooperating with police, detectives said they’ve become “extremely frustrated” in their efforts to find out how the toddler’s dismembered head, hands, and feet wound up in the Garfield Park lagoon.

Police said Kyrian’s mother, Lanisha, entrusted Kamel Harris and his girlfriend, Danyelle Foggs, to care for her son in Rockford in August, and Harris later said he gave Kyrian to three other people who said they were friends of the mother, and were picking him up.

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Chief of Detectives Dean Andrews said police know Harris has been in the Milwaukee area, and Foggs in the Chicago area, but both have been avoiding detectives.

“It’s unfathomable to me that, when we’re talking about a tender age child, that we find body parts of, and you won’t cooperate on a case like that? I just cannot wrap my brain around it,” he said.

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Andrews said Foggs and Harris are not suspects in Kyrian’s death, but witnesses who could help solve the case.

“They are two people – Kamel Harris and Danyelle Foggs – who were the last people that were with Kyrian. We have tried numerous times to contact these people, and it’s been extremely frustrating that we have not been able to make contact,” he said.

Kyrian’s severed head, hands, and feet were found in the Garfield Park lagoon on Labor Day weekend, but the body parts were not identified until two months later, after a DNA match was made.

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Harris reported the boy missing two weeks after the body parts were found, and a month after he said he’d handed the boy over to someone else.