2 PAX WERE INJURED WHEN AN ACR LGT HIT SOME TURB. THE SEAT BELT SIGN WAS ON.

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I HAD DEVIATED E OF J89; S OF IIU DUE TO WX. I HAD THE SEAT BELT SIGN ON AND MADE PA'S ABOUT EXPECTED BUMPY AIR FOR A LITTLE WHILE UNTIL WE GOT PAST LOUISVILLE. ON THE 25 MI SCALE ON MY RADAR; I WAITED UNTIL NOTHING PAINTED AHEAD; AND TURNED BTWN RETURNS. ATC RPTED LIGHT TURB WITH OCCASIONAL MODERATE. ENCOUNTERED HVY PRECIPITATION; MODERATE TURB; ALTHOUGH NO PRECIPITATION WAS PAINTING ON OUR RADAR. HIT ONE STRONG; SHARP DOWNDRAFT; ACFT FELL 200 FT BUT AUTOPLT STAYED ON. 2 PAXS CUT THEIR HEADS ON OVERHEADS -- THEY WERE NOT WEARING SEAT BELTS IN SPITE OF 2 PA'S FROM COCKPIT AND 2 OR 3 FROM FLT ATTENDANTS. WE RPTED SEVERE TURB TO ATC. WE REQUESTED PRIORITY HANDLING INTO ORD DUE TO PAX INJURY. A DOCTOR ON BOARD SAID THE PAXS'S HEAD CUTS WERE SUPERFICIAL ALTHOUGH BLOODY. I BELIEVE MY RADAR SHOULD HAVE SHOWED A RETURN; ESPECIALLY ON THE SHORTER SCALES. EARLIER; IN THE DAYLIGHT; IT GAVE US RELIABLE INFO AS WE SKIRTED WX OVER FL; AND GA. THERE WAS NOTHING ON OUR SCOPE THAT I FELT WE NEEDED TO AVOID. IF SEAT BELTS HAD BEEN FASTENED; INJURIES WOULD NOT HAVE OCCURRED. IF WE HAD SEEN ANYTHING THAT LOOKED SERIOUS ON OUR RADAR; WE WOULD HAVE AVOIDED IT. IF NOTHING SHOWS ON THE 25 MI RANGE; IT IS NOT THERE. GIVEN THE SAME INFO FROM ATC; MY RADAR; AND MY EXPERIENCE; THERE IS NOTHING I WOULD HAVE DONE DIFFERENTLY. I FEEL BAD THAT ANYONE WAS HURT ON MY AIRPLANE.