Minx planned to abscond with the money in a home-built submarine but quick police work eventually led to his arrest and imprisonment.
[2] Minx first set about building a bomb using a one-inch metal pipe, gunpowder collected from 35 12-gauge shotgun shells, a wind-up clock, and a spring mechanism.
[2] Minx needed a foolproof method of retrieving the extortion money without getting caught and a submarine, with its ability to submerge and creep away undetected from a watery drop-off point, seemed ideal.
[2] The first prototype proved inadequate to the task and was later found by the police, resting in a patch of weeds behind his parents’ home at 3725-A N. Holton St. Minx began work on a second model.
[3] The resulting craft was seven feet long, powered by automobile batteries connected to a small electric starter motor.
Moveable fins on the exterior hull enabled the sub to dive while two-gallon tin cans inside could be filled with lake water using a valve made from a kitchen sink faucet.
The cans acted as ballast tanks and helped maintain neutral buoyancy once the vessel dipped below the waves.
Hand-operated levers were used to raise and lower the diving fins and steer the rudder while a radiator petcock could be opened to regulate interior air pressure when necessary.
Minx's plan was for Davie to hire a small plane from Curtis Wright Airport (now known as Lawrence J. Timmerman Airport) at 7:30 pm on July 26, fly in a designated straight line and drop a money bundle containing $100,000 over Lake Michigan upon sighting two blinking lights (coming from Minx's sub).
The plane was to continue flying another 50 miles before turning back, giving Minx time to maneuver his submarine over to the bundle and retrieve the money.
He would then submerge for several hours before deliberately scuttling the craft off McKinley Beach and swimming to shore where his car was parked nearby.
The following day, Minx planted a small bomb in a storeroom at the Sears store on North Ave, timing it to go off when few people would be inside.
Walter decided an alternate drop-off scheme was needed so he spent several days devising a new arrangement to be executed via motorcycle.
[2] In the meantime, police were able to determine that metal fragments from the Sears store bomb blast were of a type used in ornamental ironwork.