CONROE, Texas -- Two young children and their 90-year-old grandmother are dead after a house fire north of Houston.

Authorities in Conroe say the blaze was reported at 2:30 a.m. Saturday in a house where a mother, father, their four young children and the family's grandmother were trapped inside.

The parents escaped and firefighters and neighbors helped a 5-year-old and 8-year-old escape. Officials say a 1-year-old child rescued by firefighters was pronounced dead at a hospital, and a 7-year-old and the grandmother were pronounced dead at the scene.

The parents and surviving children were treated for smoke inhalation at a hospital. Officials say heat from the fire was so hot that it melted the boot of one of the first firefighters inside the burning home.

The cause of the fire is under investigation.