Its been a while, i thought they were led up there alive.
He smothered them before putting them in the tanks. He had one alive in the car while he walked up to the hatch with the first dead one! So she watched her dad walk up there with her dead sister in his hands.
The diameter of the hatches was eight inches. The children were three and four years old. So one can think of what he had to do to get them in there.
