The police chief in Wausau says the investigation into a duplex fire that killed three young siblings is complete and the cause remains a mystery.

Chief Jeff Hardel said Friday the fire started in a bedroom near a bed where a baby was sleeping. He says investigators have ruled out a possibly overloaded extension cord as the origin. He says the cause of the fire is undetermined and no foul play is suspected.

Hardel says a smoke alarm was mounted on a bedroom wall but no one heard it go off.

The Sept. 16 fire killed 9-year-old Mitchell Melvin Block, 8-year-old Mackenzie Marie Block and 2-year-old Faith Elizabeth Fondie. Their mother, her boyfriend and four of her other children, including an 8-month-old baby, escaped from the downstairs residence.