Tom, I'm sorry for your loss.
I'd insist upon the immediate firing of the thief. I'd make a copy of the taps, and have criminal charges pressed.
Indeed, a lot rests upon the guy's first response. If he says "Oops, I thought it was ours"... and the stuff is found in his company's storeroom.... well, accidents happen.
BUT- it's best that you let the police do this questioning. You don't want to tip your hand.
Also, a thief seldom steals but once. The Police will be quite interested in the contents of his van, theft experience at his past assignments, etc.
If the Police determine that a theft has occurred, and they are the ones to approach his employer.... his employer is very likely to become your strongest ally. After all, it's likely he's had some stuff go missing as well.
