THE EMERG MED SVC (EMS/MED-EVAC) HELICOPTER (HEL) WAS ON A NGT FLT WITH A 3 MONTH OLD PATIENT ABOARD. AFTER RPRTG LVL AT3000', THE PLT TRANSMITTED HE WAS GOING TO MAKE AN EMERG LNDG. THIS WAS FLWD BY SVRL UNREADABLE TRANSMISSIONS WHICH APCHCTL PSNL THOUGHT WERE IN REF TO AN ONBOARD FIRE. AT ABOUT THE SAME TIME, A FLT NURSE TRANSMITTED ON THE HOSPITAL FREQ THAT THEY WERE ON-FIRE & WERE GOING DOWN. SOON THEREAFTER, RADAR & RADIO CTC WERE LOST. SUBSEQUENTLY, THE HEL CRASHED INA SLGT NOSE DOWN, RGT BANK ATTITUDE & BURNED. IT WAS EXTENSIVELY DMGD BY IMPACT & FIRE. CRASH DMG REVEALED THE HEL HAD IMPACTED AT A HI VERTICAL VELOCITY & WITH LITTLE OR NO FWD MOVEMENT. NO PREIMPACT PART FAILURE/MALFUNCTION OF THE ENG ORAIRFRAME WAS FND. THE HEL HAD BEEN MODIFIED (MOD) FOR EMS OPNS IAW 4 SUPPLEMENTAL TYPE CERTIFICATES (STC'S). THE MODS INCLUDED INSTLN OF A HI PRES OXYGEN (O2) SYS. DRG THE ACDNT, SVRL O2 LNS FAILED FM IMPACT OR FIRE, WHICH CREATED A BLOW TORCH EFFECT. THERE WAS NO PROVISION FOR EMERG SHUT-OFF OF THE O2 AT OR NR THE CYLINDERS. SOURCE OF FIRE WAS NOT FND.