Updated

The Chicago Fire Department says an early morning apartment fire killed two young children.

Fire officials say the victims were a 1-year-old boy and a 3-year-old girl.

Fire Department Commander Sean Flynn says the children's mother ran out of the apartment with a 6-year-old child. Flynn says the door accidentally locked behind her and she couldn't get back inside to rescue the younger children.

The fire broke out at about 3:15 a.m. Tuesday in a three-story building on Chicago's far South Side. Investigators are working to determine what caused the fire, but officials say it is believed to have started in the kitchen.