The deaths of Danielle Steiner, 34, and Aubrey Hall are being investigated as homicides. The discovery of bones Monday by a construction crew led police to dig in a backyard.
Steiner and her son were reported missing last August, although the last known contact with family was weeks earlier in June when she sent a text message, said Chief Mike Yankowski, who urged the public to share any tips.
"The smallest detail can make a huge difference in the investigation," the police department said in a statement.
During a news conference, Yankowski deflected most questions about details of the investigation. He said Steiner had lived at two addresses on the street where the remains were found.
"Obviously it's very traumatic for them," Yankowski said of relatives. "It's a very tragic case, especially when you're dealing with a 5-year-old."
No arrests have been made. The Lansing State Journal, citing court records, said the boy's father was granted full custody of Aubrey in June 2018. He claimed Steiner was using drugs, although she denied it.
A state child welfare investigator said Steiner had moved to a neighbor's home because she couldn't afford rent, according to court records. When the investigator tried to contact her again, he couldn't find her.