CHICAGO (CBS) — Police and the Lake County Coroner’s office were investigating, after a Highland Park couple died of carbon monoxide poisoning in their home earlier this week.

A Highland Park Fire Department lieutenant found the bodies of his parents – 79-year-old Pasquale Fontanini and 76-year-old Rina Fontanini – in their home Monday morning. Pasquale Fontanini’s body was on the first floor, and Rina Fontanini’s body was in an upstairs bedroom.

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The Lake County Coroner’s office said autopsies determined both husband and wife died of carbon monoxide poisoning.

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Police said investigators believe Pasquale Fontanini heard the home’s carbon monoxide detector going off, found the family car’s keyless ignition running in the attached garage, and opened the garage door, but the deadly carbon monoxide did not dissipate quickly enough, and the couple was overcome.

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Their son tried to perform CPR after finding their bodies, but could not revive them.