THE NOSE GEAR OF A SENECA RETRACTED DURING LANDING ROLL; CAUSING DAMAGE TO THE AIRCRAFT.

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AFTER OUR 5TH TOUCH-AND-GO I REQUESTED A FULL STOP. EACH TOUCH-AND-GO THE GEAR WAS ACTUATED UP AND DOWN AND THE SAME STANDARD CALLS WERE ANNOUNCED. WE WERE CLRED TO LAND BY THE TWR IN DOWNWIND. I PERFORMED A GEAR DOWN BEFORE LNDG CHKLIST AND BOTH I AND MY FLYING FRIEND CONFIRMED 3 GREEN. WE ALSO CONFIRMED THE NOSEWHEEL WAS DOWN VIA THE MIRRORS ON EACH ENG NACELLE. AFTER WE TURNED FINAL WE DSNDED THROUGH 500 FT AGL AND I ANNOUNCED GEAR DOWN AND STABILIZED. WE FLARED AND THE MAINS TOUCHED DOWN SOFTLY IN THE TOUCHDOWN ZONE. THE NOSEWHEEL TOUCHED DOWN SOFTLY AND WE ROLLED FOR APPROX 150 FT THEN THE NOSE GEAR RETRACTED AND THE NOSE SLOWLY IMPACTED THE RWY. THE PROPS; WHICH WERE STILL SPINNING; DIDN'T HAVE CLRNC FROM THE GND AND CURLED BACK AS THEY BOTH IMPACTED THE RWY. THE ENGS BOTH SHUT DOWN ON THEIR OWN AS WE COASTED DOWN. THE R SEAT PLT PULLED OFF THE MIXTURE LEVERS AND THE FUEL SELECTORS TO MINIMIZE POSSIBLE FIRE HAZARDS. I; IN THE L SEAT; TURNED OFF THE MAGNETOS; ALTERNATORS; AND MASTER SWITCHES. BEFORE I LEFT THE ACFT; COPLT TURNED ON THE MASTER AND I RADIOED TO TWR TO CLOSE THE RWY AND THAT WE HAD A LNDG GEAR MALFUNCTION. WE THEN BOTH NOTICED THAT THE GEAR INDICATORS WERE STILL INDICATING 3 GREEN AND THE LNDG LEVER WAS IN THE DOWN POS. WE VACATED THE ACFT AND IMMEDIATELY BEGAN VISUALLY INSPECTING THE DAMAGE. WE THEN RETRIEVED A DIGITAL CAMERA TO DOCUMENT THE EVENTS. NEXT WE WALKED DOWN THE RWY TO SEE IF THERE WAS SOME INDICATION OF WHY THE NOSEWHEEL FAILED AND TO TAKE MORE PICTURES. AS WE WALKED BACK TOWARDS THE ACFT; 3 INDIVIDUALS SHOWED UP AND BEGAN JACKING UP THE ACFT AND WE NOTICED THAT THE COCKPIT DOOR WAS OPEN. WE YELLED AT THEM TO GET AWAY FROM THE ACFT SINCE THE NTSB HADN'T RELEASED THE ACFT YET. THEY TOLD US THAT THEY DIDN'T CARE AND THE TWR TOLD THEM TO MOVE THE ACFT. WE THEN KEPT THEM AWAY AND CALLED THE OWNER OF THE ACFT. WE CLEANED OUR PERSONAL GEAR OUT OF THE ACFT. THESE ITEMS CONSISTED OF FLT BAGS; WATER; AND ACFT MAINT ITEMS. AFTER THESE EVENTS; I STOOD BACK AS A BYSTANDER AND DOCUMENTED THE FOLLOWING EVENTS WITH MY CAMERA.