AFTER LNDG B737 CREW IS INSTRUCTED TO HOLD SHORT OF RWY 32 BUT INADVERTENTLY CROSS.

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WE HAD JUST COMPLETED THE ILS TO RWY 36 AND AS WE CLRED THE RWY; WE WERE GIVEN INSTRUCTIONS TO TAXI VIA L TURN ON TXWY S; L ON TXWY F AND HOLD SHORT OF RWY 32L. I READ BACK THE INSTRUCTIONS AND VERIFIED THE TAXI PROGRESS AS THE CAPT TURNED THE ACFT ONTO TXWY S AND THEN TXWY F. AT THAT POINT; I WAS SATISFIED THE CAPT UNDERSTOOD OUR TAXI INSTRUCTIONS AND WOULD HOLD SHORT OF RWY 32L. I BEGAN THE FLAPS UP/CLEAN-UP FLOW. JUST AS I WAS COMPLETING THE FLOW; TWR CALLED AND INSTRUCTED US TO HOLD SHORT OF RWY 32L. I REDIRECTED MY ATTN OUTSIDE AND VERBALLY REPEATED 'HOLD SHORT' TO THE CAPT. AT THAT TIME WE HAD ALREADY TAXIED OVER THE HOLD SHORT LINE AND THE NOSE OF THE ACFT WAS ON THE RWY. REALIZING WE WERE ALREADY ON THE RWY; THE TWR CTLR INSTRUCTED US TO CLR THE RWY AND CONTACT GND CTL ON THE RAMP SIDE OF RWY 32L. WE THEN TAXIED TO THE GATE AND SHUTDOWN. SEVERAL FACTORS CONTRIBUTED TO THIS EVENT. I SHOULD HAVE NEVER ASSUMED THE CAPT UNDERSTOOD ALL OF OUR CLRNC. I BELIEVE THE CAPT HAD HEARD THE FIRST PART OF THE TAXI CLRNC AND EITHER DID NOT HEAR OR JUST FORGOT TO HOLD SHORT OF RWY 32L. IN ADDITION; MY ATTN WAS DIRECTED INSIDE THE ACFT AT THE WRONG TIME. I SHOULD HAVE MAINTAINED A SCAN OUTSIDE THE ACFT AND VERIFIED WE WERE GOING TO HOLD SHORT AS INSTRUCTED. I USUALLY VERBALLY REPEAT ALL TAXI INSTRUCTIONS TO THE CAPT. THIS TIME I DID NOT AND I CAN'T EXPLAIN WHY. ALSO; WE WERE OPERATING ON 9 HRS 50 MINS OF CREW REST BTWN OUR FLTS. I BELIEVE THE CAPT WAS FEELING FATIGUED; AS EVIDENCED BY THE FACT HE PULLED HIMSELF FROM THE TRIP FOLLOWING THE NEXT LEG. IN THE FUTURE; I INTEND TO MAINTAIN MORE OF A SCAN OUTSIDE THE ACFT DURING CRITICAL TIMES OF TAXIING. I SHALL PAY MORE ATTN TO DESIGNATED HOT SPOTS ON THE 10-9 PAGE. FOR EACH LEG; I SHALL WRITE DOWN ALL TAXI INSTRUCTIONS; READ BACK THESE INSTRUCTIONS TO ATC AND THEN REPEAT THESE INSTRUCTIONS ONCE AGAIN TO THE CAPT AND ENSURE I RECEIVE VERBAL CONFIRMATION.