Anthony Marcus, 53, was arrested Sunday. Police said they found a dog dead in the home along with the bodies of a 50-year-old woman and 17-year-old girl.
Cmdr. Joe Florip, of the Waukegan police department, said officers who came to the house Sunday evening found what appeared to be a suicide note. Marcus was wounded but able to walk, Florip said.
"This isn't a whodunnit," Florip told the Chicago Sun-Times. "We're not looking for an offender. We're classifying this as a domestically related."
Kayla Walker, a neighbor, said the family had lived in their home in Waukegan, a city 40 miles north of Chicago on Lake Michigan, for about eight years. She described them as friendly.
"This is just shocking," she said. "Their daughter was in a wheelchair. That's why it's so shocking."