Investigators think that botanical evidence found on the shoes of Sean Phillips could lead them to the body of his daughter, Katherine, who was last seen when she was 4-½-months-old, Ludington Police Chief Mark Barnett said at a news conference.
Phillips was last year convicted of unlawful imprisonment in the case and sentenced to 10 to 15 years in prison. Police are treating Katherine's disappearance as a homicide.
Police will resume the search for Katherine's remains June 28-29, because it's the same timeframe in which she disappeared in 2011 and similar plants will be present, Barnett said.
Authorities have been working with scientists from Michigan State University, University of Michigan and the Field Museum in Chicago to retrieve and identify grass-like plants called sedges, sphagnum moss, pine, sand from a dune and other elements, he said.
Detective J.B. Wells wrote in a letter to prospective applicants to assist in the search this June that the goal is to identify where Phillips walked, which could show them "where he put Baby Kate."
Police have declined to identify specific areas where they plan to search, but say locations with a high probability of containing all of the elements have been established.
Investigators say people with knowledge of plants should apply by May 1, and those who are accepted will be trained in advance. Wells called it an "incredible opportunity" for biologists to "use their skill and education to help solve a criminal case and bring justice to Baby Kate."
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.