Tire Sidewall problems of the 2000 Ford E-350

Was making an emergency transport with a cardiac patient on board, aprox speed @ 70 mph, felt viberation for 5 to 10 seconds, then tread of tire tore from sidewalls and tread flew from under ambulance, came to a slow stop noticed what had actually occured, called for mutual aid to transport cardiac patient on to hospital, took pictures of tread, side walls remaining on rim, slowly drove ambulance back to bp station where attendant changed tire. Reported incident to firestone dealer, danville, ky 40422 (dot number: vd1l1x1469 tiresize: lt22575r16).