AT 1027 THE PLT RECEIVED A WX BRIEFING FOR HOLBROOK, AZ. THE PLT WAS TOLD OF FORECASTS FOR "AT LEAST MODERATE ICING IN CLOUDS OR IN PRECIPITATION ABOVE THE FREEZING LEVEL" & THE BRIEFER STATED "I GUARANTEE YOU MODERATE TO SEVERE TURBULENCE..." FOR A ROUTE INTO HOLBROOK. THE PLT LATER GOT WX INFO FOR AMARILLO, TX & FILED AN IFR FLT PLAN TO AMARILLO. THE ACFT DEPARTED AT 1150. AT 1204 THE PLT REPORTED HE HAD AN ENG PROBLEM. RADAR PLOTS INDICATE THE ACFT WAS MAINTAINING A CLIMB OF ABOUT 500 FPM UP TO 11,200 FT. THIRTY SECONDS LATER THE ACFT HAD DESCENDED TO 8,500 FT. RADIO & CONTACT WAS LOST ABOUT 1207. BASED ON A WX STUDY THE PROBABLE METEOROLOGICAL CONDITIONS THAT EXISTED ABOUT THE TIME & IN THE AREA OF THE ACCIDENT WERE MODERATE TO SEVERE TURBULENCE BELOW 12,000 FT MSL & WHILE IN THE CLOUDS ABOVE FREEZING LEVEL THE ACFT LIKELY ENCOUNTERED MODERATE TO SEVERE RIME ICING. THE ACFT WAS NOT CERTIFICATED FOR FLT INTO KNOWN ICING CONDITIONS. THE FLT WAS TRANSPORTING PRISONERS FOR VARIOUS GOVERNMENT AGENCIES.