VIENNA (AP) — Austrian police investigating a grenade blast were less puzzled by the explosion and more by where it took place — inside a wood stove that appeared to contain nothing but firewood.

The woman who owned the wood burner also had no clue at first. After all, she only put firewood inside.

Or so she thought.

Police spokeswoman Petra Datscher said Monday a World War II grenade apparently landed on a tree during fighting. It was then enveloped by wood growing around it to the point that it was invisible when the tree was chopped down for firewood and sold to the supermarket where the unidentified 22-year-old woman bought it.

The blast Sunday in the lakeside town of Gmunden shattered the stove's glass panel, but the sturdy wrought-iron stove prevented injuries.