Officials say they've interviewed several witnesses this week in the investigation into the 2011 disappearance of Baby Kate.
Last year, Sean Phillips, the child's father, was convicted of unlawful imprisonment in the case and sentenced to prison. Police are treating his daughter Katherine's disappearance as a homicide. She was last seen when she was 4 1/2 months old.
WOOD-TV is reporting the interviews Tuesday were at the request of Mason County prosecutors. Details weren't released.
Katherine lived with her mother, Ariel Courtland, in Ludington, about 80 miles northwest of Grand Rapids.
The child hasn't been found despite extensive searches. Police have said they plan to resume the search for Katherine's remains June 28-29. Officials received more than 100 applications to be a part of what prosecutors are referring to as a "botanical search."
Investigators think that extremely rare botanical evidence found on the shoes of Sean Phillips could lead them to the body.
"The location that would support all of these elements is so rare and specific to the surrounding area, that it is only a matter of time before the exact location is found," Ludington Police Chief Mark Barnett said at an April press conference.A letter, apparently written by Sean Phillips from prison, detailed what happened the day Baby Kate disappeared. The five page hand-written letter explained how 4-month-old Baby Kate died and where her body was placed. Phillips said he left Baby Kate's body in a "peaceful place."