KANSAS CITY, Mo. (AP) _ A fire killed six family members gathered for a girl's birthday party Friday in a tragedy investigators said could have been prevented if the smoke detectors in the house had had batteries.

Six other people were injured, including the girl's father, who was critically hurt with burns over 90 percent of his body.

Firefighters said the batteries had been removed from two smoke detectors in the small, two-story house.

``The most tragic part of this fire is that a working smoke detector would have alerted them to the fire before it became fatal,'' said Domenic Cerrone, battalion chief.

The fire started near a mattress just inside the front door, said Brad Humston, Fire Department spokesman.

Investigators had not determined the cause.

Those killed _ the birthday girl, her mother, an aunt and three young cousins _ were upstairs when the blaze started, Humston said.

Their names and ages were not immediately released.

The parents of the birthday girl had asked the others to the house to celebrate her birthday the night before.