Axle Assembly problems of the 2001 Toyota Prius

At 45mph on dry pavement, clear weather conditions I disengaged cruise control by tapping brakes. So that I could accelerate into traffic. The vehicle pulled hard to the left. I then found myself in a counterclockwise skid across the pavement as I left the pavement I felt the rear of the vehicle drop. Now spinning I bounced off a tree and into a drainage ditch with about 1 and 1/2 foot of mud and water. It was then I noticed that one of the wheels (left rear) had separated from the vehicle and was on the other side of the ditch. It all happened so fast I have no idea of the actual cause. The studs which hold the wheel to the axel shearedoff, ans axle snapped, the undercarrage and supports were tangled. Fhp officer at site indicated it could have been mechanical malfunction of some sort. Because of conditions I contacted Toyota as well as insurance CO (progressive). No one has yet evaluated the cause. Toyotas first comment was "what do you expect from an investigation" my answerwas " to prevent a similar event in the future". As I related the event to every one I also noted that the steering didnot let me correct the skid. The vehicle is currently held in limbo by insurance and Toyota. While I am walking. All safety equipment functioned properly.