Bicycle detail

My bike was locked to a free standing mailbox post made of 4x4in wood with the actual mailbox on top. I locked the rear wheel and bike frame to the post via one U lock, and the front wheel to the U lock via steel cable. All of this was covered by a protective sheet made specifically for bicycles. The mailbox is farther in from the property line; it is about 3ft from the front porch and about 20-30 ft from the street.
This morning around 9am, I was notified about my bike being stolen. In disbelief I quickly went outside and saw the mailbox severed from the post, and my bike missing. It looks like the thieves broke off the mailbox from the post, and lifted the bike from the post and went away with it. They then later dropped the mailbox a couple feet down the driveway. My bike has been missing since.