A B777 IN DESCENT AT FL 280 DECLARED AN EMER AND SHUTDOWN THE L ENGINE DUE TO UNKNOWN FLUID STREAMING OFF THE L WING.

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AT APPROX 1 HR 50 MINS AFTER TKOF; POS APPROX 70 NM E OF KLAX; FL 280; WE RECEIVED A CALL FROM THE PURSER. HE SAID A PAX (A DEADHEAD ACR PLT) WAS OBSERVING A FLUID LEAK ON THE L WING. THE CAPT AND I DISCUSSED THE POSSIBLE CAUSE AND CHKED THE SYSTEMS SYNOPTICS. THE ONLY POTENTIAL ABNORMALITY NOTED WAS A HIGHER FUEL FLOW AND LOWER FUEL QUANTITY ON THE L SIDE. THE CAPT DIRECTED ME TO PERFORM A VISUAL INSPECTION. MY INSPECTION REVEALED FLUID STREAMING OVER THE ENTIRE L WING. THE FLUID APPEARED TO BE CLR IN COLOR AND WAS STREAMING OFF THE TRAILING EDGE AT SEVERAL POINTS. I OBSERVED THE L WING FROM DOORS 3 AND 4L AS WELL AS FROM BUSINESS CLASS IN AN ATTEMPT TO ISOLATE THE SOURCE OF THE FLUID. I WAS UNABLE TO DETERMINE THE SOURCE. THE COMPANY PLT WHO INITIALLY RPTED THE POSSIBLE LEAK IDENTIFIED HIMSELF AND STATED HIS CONCERNS. I RETURNED TO THE FLT DECK WITH THE PURSER TO DISCUSS THE PROB. ON THE WAY BACK TO THE FLT DECK I OBSERVED THE R WING AND NOTICED NO ABNORMALITIES OR FLUID ON THAT WING. I RPTED MY OBSERVATION TO THE CAPT. AT THAT POINT WE WERE INSIDE OF CIVET ON THE CIVET 4 ARR. WE FIGURED THAT THE ONLY POSSIBLE FLUIDS COULD BE FUEL; OIL OR HYDRAULIC. SINCE IT APPEARED TO BE CLR WE THOUGHT THE FUEL WAS THE MOST POSSIBLE SOURCE. AT THAT POINT THE DECISION WAS MADE TO DECLARE AN EMER AND LAND AT LAX. THE PURSER WAS INFORMED TO PREPARE THE CABIN FOR AN EVACUATION WITH AN ETA TO LNDG OF APPROX 10 MINS. BEING UNABLE TO PRECISELY ISOLATE THE SOURCE OF THE FLUID; BUT WITH A CONCERN OF FIRE IF THE FLUID WAS FUEL; A PRECAUTIONARY ENGINE SHUTDOWN WAS DIRECTED AND ACCOMPLISHED. THE L ENGINE WAS SHUT DOWN REFERENCING BOTH THE SEVERE DAMAGE AND FUEL LEAK PROCS. THE CAPT ASKED SOCAL APCH TO NOTIFY OUR COMPANY SINCE WE WERE WITHIN 10-15 MINS OF LNDG. AFTER ACCOMPLISHING DESCENT AND APCH CHK LIST I CALLED THE FLT ATTENDANT AT DOOR 4L. SHE TOLD ME THAT THE FLUID STREAM HAD STOPPED. WE ADVISED THE PURSER AND LAX TWR THAT WE PLANNED TO LAND ON RWY 25L; CLR THE RWY AND HAVE THE FIRE DEPT PERFORM A VISUAL INSPECTION. THE CAPT PERFORMED A SINGLE ENGINE ILS/VISUAL APCH ON RWY 25L. WE EXITED THE RWY; ADVISED THE CABIN TO REMAIN SEATED AND TOLD THE PAX THAT FIRE VEHICLES WOULD INSPECT THE JET. THE FIRE DEPT RPTED THERE WAS NO FLUID LEAKING AND WE WERE CLRED TO OUR GATE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AIRPLANE HAD BEEN DEICED TWO HRS PRIOR TO DEP FOR OVERNIGHT FROST. THE RPTR SAID THE WALK AROUND CHK REVEALED NO INDICATION THE AIRPLANE HAD BEEN DEICED WITH NO DRIPPING OR WET FLT SURFACES. THE RPTR STATED IT WAS A WARM BRIGHT SUNNY DAY WITH NO INDICATION DEICING FLUID WOULD BE REQUIRED. THE RPTR SAID THE AIRPLANE HAD FLOWN FOR THREE HRS WITH NO INDICATION OF FLUID COMING OFF THE L WING. THE RPTR SAID TEN MINS PRIOR TO ARR THE FLUID WAS RPTED STREAMING OFF THE L WING AND VISUALLY VERIFIED BY THE RPTR. THE RPTR SAID WHEN ON THE GND; MAINT INSPECTED THE AIRPLANE AND DETERMINED THE FLUID WAS DEICING FLUID BY ITS COLOR. THE RPTR STATED THIS IS THE THIRD EVENT OF A B777 ENGINE BEING SHUTDOWN FOR AN APPARENT FUEL LEAK CAUSED BY DEICING FLUID TRAPPED IN TRAILING EDGE AND LEADING EDGE CAVITIES. THE RPTR SAID FOR THIS TYPE OF DEICING NO DOCUMENTATION IS REQUIRED TO BE GIVEN TO THE CREW.