San Luis Obispo, CA – Investigators now know the cause of an explosion that set a house on fire.

Yesterday’s flames burned the Arroyo Grande home completely to the ground.

Let’s begin with the facts first.

The fire started at two o’clock in rural Arroyo Grande.

Witnesses reported hearing a loud explosion and then seeing the house consumed by black smoke.

Earlier in the day, the gas company was at the house filling up a propane tank.

Investigators say, the cause of the fire is officially connected to an open valve on that tank.

Cal Fire Battalion Chief Andy Anderson says “This is truly the definition of an accident. There was a propane company that came out earlier in the day. They did fill the propane tanks at some point the propane was bleed off and so that way they could force any of the air out of it and it appears that that line was not closed off.”

The open valve allowed gas to build up inside the garage turning it into a ticking time bomb.

“You have an explosive amount of fuel in here that, all it’s doing is looking for an ignition source and it has to get the right oxygen and fuel mixture and once it gets that this is exactly that. It’s just a, basically a gigantic bomb that went off ” said Anderson.

The homeowners were in the finishing stages of redecorating.

Just yesterday, this is what the bedroom looked like before the fire broke out.It’s now a charred soggy mess.

No one was home at the time of of the explosion so it’s unlikely anything could have been done to prevent it.

Fire officials say there are measures you can take to prevent something like this from happening to you.

According to Anderson, “They have caps that you can put on the pipes and so you could screw it on here and that this isn’t going to be open. That way, this is just an open oraphus so once that valve gets cracked it’s just going to start flowing gas.”

It’s also a state law that your water heater is lifted at least 18 inches off the ground.

“The hope is getting it off the ground the propane will be able to evacuate and so that way you won’t have an explosion like this”, said Anderson.

Fortunately the home served as a second residence for the owners, who say they will rebuild.