LUDINGTON, Mich. -

A West Michigan man has been sentenced to 10 to 15 years in prison for the disappearance of his infant daughter, and police say they're now treating the case as a killing.

Mason County Circuit Judge Richard Cooper in Ludington sentenced 22-year-old Sean M. Phillips of Victory Township for unlawful imprisonment.

Katherine Phillips was 4 1/2 months old when she was last seen in June 2011. She lived with her mother Ariel Courtland in Ludington, about 80 miles northwest of Grand Rapids.

The child hasn't been found despite extensive searches.

After the sentence, Ludington police Chief Mark Barnett said the case now is being investigated as a homicide by his department, the FBI, state and county authorities.