State investigators determined Monday that a propane gas leak under their home caused the deaths of Amber Smith, 20, and her 4-month-old son Simon Smith, who died in the Saturday afternoon in a thundering explosion that flattened their home.

Their home was located at 3809 County Road 144 near Calwood in Callaway County.

Autopsies were performed Monday by the Boone/Callaway County Medical Examiner’s Office.

Callaway County Sheriff Dennis Crane said preliminary results of the autopsies indicated the mother and her son’s injuries were consistent with the explosion and there is no indication of previous injuries or foul play. He said there also is no indication of criminal activity related to the explosion.

There was no fire at the home when authorities arrived at the scene.

The State Fire Marshal’s Office investigated the explosion on Monday. Backhoe equipment was used to move the debris around to determine the cause of the explosion.

Crane said indications are that the explosion was caused by a propane gas leak under the home.

“The Fire Marshal’s investigators determined the explosion was caused by a propane gas leak underneath the house. They found a copper gas line that had been capped. There was corrosion near where it had been capped to branch the line to another part of the house. The corrosion indicates gas leaked for a long time and it gradually built up under the house. The explosion apparently was caused by a spark from the kitchen stove. They found a split propane pipe near the stove,” the sheriff said.

Crane said the home was owned by Delbert Smith, the husband and father of the explosion victims. He said Smith is employed by the City of Fulton. The home is located near Calwood but has a Williamsburg mailing address.

Crane said Smith had left only a few minutes before the explosion to go into Fulton to get some things for the home.

The sheriff said investigators tell him it is not unusual for people to become accustomed after a period of time to the odor that is added to gas and not to notice it.

Crane said it also is likely that most of the gas was trapped underneath the house and collected there because propane gas is heavier than air and settles to the lowest point possible.

The big collection of gas under the house rather than inside the house would not be noticed as easily.

Crane said Smith told investigators that he had detected a slight odor in the house earlier but he wasn’t sure it was propane gas.

“It’s a tragic situation for a young man to lose his young wife and child,” Crane said.