Police said the fire broke out around midnight in the 3300-block of Swanson Court Apartments in Portage.
One person was rescued from the apartment building, but a second person, 87-year-old Maria Leal, died from her injuries.
Police say a K-9 detected flammable liquids at the scene, and a suspect allegedly made threats to start the fire.
Police impounded the suspect's vehicle and executed a search warrant for it.
That person is being held on unrelated charges on Tuesday night while the investigation continues.