VIOLATION OF FAR 91 PT 111 BY A UNITED STATES CUSTOMS ACFT IN PURSUIT OF ITS NORMAL DUTIES.

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AFTER COMPLETING OUR SEARCH AND RESCUE MISSION; WE TURNED N TOWARDS OPF. AFTER APPROX 20 MINS IN CRUISE; THE CAPT LOOKED TO HIS L AND NOTICED OUR AIRPLANE'S SHADOW; ALONG WITH THE SHADOW OF ANOTHER ACFT IN EXTREMELY CLOSE PROX TO OUR OWN. THE CAPT THEN ASKED ME TO LOOK AT OUR 5-6 O'CLOCK POS AND DETERMINE WHETHER OR NOT THERE WAS AN ACFT IN CLOSE PROX TO US. WHILE LOOKING BACK TOWARDS OUR TAIL SECTION; I SAW THE R WING OF ANOTHER AIRPLANE WHICH APPEARED TO BE TRAVELING DIRECTLY BEHIND US; AT THE SAME ALT; AND NO MORE THAN 50 FT AWAY. THE CAPT SEEMED CONCERNED ABOUT THIS UNIDENTED ACFT. IN ORDER TO TRY TO MAKE THIS ACFT MOVE AWAY; THE CAPT INITIATED A SERIES OF SMALL ROLLS IN OPPOSITE DIRECTION (ROCKING THE WINGS). AFTER THIS ACTION; THE ACFT MOVED TO OUR 5:30 POS; WHERE I OBSERVED THAT THE ACFT FOLLOWING US WAS A TWIN TURBOJET PWRED AIRPLANE; PRESUMABLY A CESSNA CITATION OPERATED BY UNITED STATES CUSTOMS. I INFORMED THE CAPT OF MY OBSERVATION; AND HE INSTRUCTED ME TO CONTACT OUR GND CTLR. I INFORMED THE GND CTLR OF OUR SIT; AND ASKED HIM TO RELAY OUR SIT TO THE UNITED STATES CUSTOMS OFFICE. DURING THE INTERCEPT; THE AIRPLANE (ASSUMED NOW TO BE CUSTOMS); REMAINED AT OUR 5:30 POS IN DANGEROUSLY CLOSE PROX. AFTER APPARENTLY READING OUR TAIL NUMBER; THE ACFT DISAPPEARED. THE AIRPLANE WAS IN CLOSE PROX FOR APPROX 2 MINS THAT WE KNOW OF. AS A PRECAUTIONARY MEASURE; WE ASKED OUR GND CTLR TO INFORM OPF TWR THAT WE WOULD BE LNDG WITH AN INOP XPONDER. WHEN WE APCHED OPF AIRSPACE; THE ATC CTLR CONFIRMED THAT OUR XPONDER WAS INOP. AFTER LNDG; OUR GND CTLR CONFIRMED THAT THE ACFT WHICH INTERCEPTED US WAS A UNITED STATES CUSTOMS ACFT. THE AIRPLANE WHICH INTERCEPTED US DID NOT ATTEMPT AIR-TO-AIR CONTACT ON 122.75 MHZ; NOR DID THEY GIVE HAND SIGNALS OR ACFT MOTION SIGNALS AS PRESCRIBED IN AIM PARAGRAPH 5-90; 5-91; OR TABLE 5-93 OF THE SAME. IN ADDITION; THEY DID NOT ATTEMPT ANY SORT OF COM. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR FLIES THE CESSNA SKYMASTER FOR A VOLUNTEER ORGANIZATION THAT HELPS IN THE SEARCH AND RESCUE OF CUBAN BOAT PEOPLE. HIS ORGANIZATION CALLS CUSTOMS BEFORE DEPARTING FOR A SEARCH MISSION AND THE PLTS ARE ON A CORDIAL BASIS WITH CUSTOMS. WHEN THEY FIND A BOAT OR RAFT; THEY DROP SMOKE; THEN A RADIO TO ESTABLISH CONTACT. THEY CALL THE COAST GUARD FOR ASSISTANCE IF NECESSARY. THE RPTR DOES NOT LIKE FINDING A CUSTOMS ACFT ON HIS TAIL. THIS HAPPENED AGAIN IN THE SAME ACFT JUST 2 WKS AFTER THE RPTED INCIDENT. THE FAULTY XPONDER WAS FIXED THE SAME DAY AS THE RPTED INCIDENT. THE RPTR HAS NOT HEARD FROM THE FAA ON THIS MATTER.