CARGO AIR TAXI ACFT 'ATTACKED' BY GOV AGENCY ACFT ALLEGEDLY PRACTICING INTERCEPT TECHNIQUES.

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I WAS IN CRUISE FLT; 10000 FT; IFR FLT PLAN. APPROX 22 MI SE OF JULIAN VOR; A TA WAS ISSUED BY ZLA; TFC IN SIGHT. CTLR MADE THE COMMENT THAT THE ACFT POSSIBLY WANTS TO PULL UP ASIDE AND TAKE A LOOK AT YOU. I REPLIED ROGER. THE ACFT WAS AT THE TIME PARALLEL TO MY COURSE A MI OR SO OFF MY L WING. SHORTLY THEREAFTER; ACFT WAS APCHING ME AT A 45 DEG ANGLE AT MY ALT. IT WAS THEN I REALIZED THERE WAS NO APPARENT MOTION OF THE ACFT TO ALTER HIS COURSE TO AVOID A MIDAIR. I ROLLED SHARPLY TO R; PITCHED DOWN AND MISSED. I ROLLED LEVEL; THE OTHER ACFT WAS JUST NOW TAKING EVASIVE ACTION. HE ROLLED L AND PITCHED DOWN. WE MISSED BY ABOUT 50-75 FT VERT AND HORIZ. I WAS UNDER THE ASSUMPTION THE ACFT APCHING ME WAS IN CTL. IT WAS IN THE LAST SECONDS I REALIZED IT WAS NOT AND EVASIVE ACTION HAD TO BE TAKEN. AFTER TALKING TO ZLA ON THE GND; I DISCOVERED THE ACFT IN QUESTION WAS A UNITED STATES (GOV AGENCY) ACFT POSSIBLY PRACTICING INTERCEPTS BUT THIS COULD NOT BE CONFIRMED. CTR SAID THEY WERE NOT IN CONTACT WITH THE ACFT BUT THEY KNEW IT WAS UNITED STATES (GOV AGENCY) ACFT BY THE XPONDER CODE. IF THE ACFT WAS IN FACT PRACTICING INTERCEPTS; WHY WAS I NOT ADVISED AND WHY DID THE SIT PROGRESS TO NEAR DISASTER? A NEAR MIDAIR COLLISION RPT WAS FILED WITH ZLA. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES HE FILED A RPT WITH ATC AND WITH LCL FSDO OFFICE. HIS BOSS CALLED THE MAIN BOSS AT HEADQUARTERS TO ALSO FOLLOW UP ON THIS SIT. THE COMPANY PERSONNEL ARE FURIOUS. RPTR CONTACTED ATC BY PHONE AND WAS INFORMED THAT THIS WAS A GOV AGENCY ACFT AND CTLR COULD NOT SAY MUCH ON THE AIR. AGENCY ACFT HAVE A DISCRETE SQUAWK CODE THEY USE; BUT ARE NOT UNDER ATC CTL. 'THEY KIND OF DO THEIR OWN THING OUT THERE;' AND THEY ALLEGEDLY PRACTICE INTERCEPT TECHNIQUES. THE NEXT DAY AFTER DISCUSSING WITH COMPANY; RPTR CALLED AND SPOKE WITH SUPVR AT ATC. HE WAS INFORMED THAT AN ACR HAD ALSO FILED A MIDAIR RPT IN THAT SAME AREA WITH THE SAME SCENARIO A COUPLE OF DAYS EARLIER. RPTR STATED THAT IS AS CLOSE AS HE EVER WANTS TO COME TO ANOTHER ACFT. HE THOUGHT THEY WERE SURELY GOING TO HIT EVEN WITH HIS EVASIVE MANEUVERS AS THE AGENCY ACFT WAS CLOSING SO FAST. HE AND HIS COMPANY ARE BOILING MAD AND WANT SOMETHING DONE. AGENCY SHOULD NOT USE CIVILIAN ACFT TO DO THEIR PRACTICING.