Police would not immediately confirm the third death, but the Cook County medical examiner's office did.
Hoffman Estates Police Lt. Rich Russo earlier said a woman called authorities early Friday reporting someone had been stabbed. Officers found two people stabbed to death and three others wounded at the home.
Russo said investigators are confident the stabbings did not result from a home invasion and that the killer or killers were among those wounded.
Russo declined to identify any of the five adults but said they all knew each other.
Hoffman Estates is 28 miles northwest of Chicago.
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