ATF agents join local probe into Chicago fire that killed 4

CHICAGO (AP) - Federal agents have joined Chicago police and fire investigators in trying to determine the cause of a suspicious fire that killed four, including three children.

The Cook County medical examiner's office said Wednesday the youngest victim, 3-month-old Melanie Watson, died of injuries suffered when her father leaped with her from their burning third-floor apartment. Her death was ruled a homicide.

Officials say the autopsies of the two other children, ages 7 and 4, are complete but results are being withheld pending formal identification.

The death of the fourth victim, 56-year-old Kirk Johnson, was also ruled a homicide.

Officials say Bureau of Alcohol, Tobacco, Firearms and Explosives agents joined the investigation after findings thought to pinpoint the cause of the blaze were inconclusive.

Chicago police say they're questioning a person of interest.