RWY INCURSION BY A DC9 FLT CREW WHEN CLRED TO TAXI TO RWY 21R FROM INTXN F AND INTXN PP AT DTW; MI.

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WE DEPARTED THE GATE AND WE WERE INSTRUCTED TO TAXI TO RWY 21R VIA THE BLUE-8 TAXI RTE. WHILE WE WERE ON TXWY PP AND APCHING THE INTXN WITH TXWY F; WE WERE TOLD TO STOP. THERE WAS AN RJ THAT STOPPED SHORT OF THE HIGH SPD TXWY; AND WAS CLRING RWY 21L ON TXWY F. I ASSUMED THAT WE WOULD ALLOW THE RJ TO PROCEED BY US ON TXWY F; AND THEN WE WOULD CONTINUE TO RWY 21R VIA THE BLUE-8. WELL; WE WERE ISSUED NEW TAXI INSTRUCTIONS AT THIS TIME. WE WERE TOLD TO TAXI TO RWY 21R VIA TXWY F. WE WEREN'T INSTRUCTED TO HOLD SHORT OF RWY 21R. AS WE APCHED RWY 21R; WE RECEIVED IN QUICK SUCCESSION SEVERAL ACARS MESSAGES. THE FIRST MESSAGE TITLE READ; 'YOU ARE ILLEGAL.' THIS GOT OUR ATTN; AS WE COULDN'T UNDERSTAND WHAT WAS UP. THE FO PULLED UP THIS MESSAGE TO SEE WHAT WAS GOING ON. WE THEN RECEIVED ANOTHER; AND THEN ANOTHER ACARS MESSAGE. IN TOTAL; WE RECEIVED ABOUT 4 MESSAGES FROM THE TIME WE TURNED ONTO TXWY F. AS WE APCHED THE HOLD SHORT LINE; WE HEARD GND CTL TELLING US TO STOP. HIS WORDS WERE SOMETHING TO THE EFFECT OF; 'ACR X; STOP.' WE INSTANTLY STOPPED; AND QUICKLY LOOKED TO OUR R AND NOTICED A CRJ TAKING OFF. WE DEFINITELY WEREN'T ON THE RWY; BUT WE MAY HAVE BEEN OVER THE HOLD SHORT LINE. GND THEN CLRED US TO TAXI TO RWY 21R VIA TXWY F; RWY 9L AND TXWY M TO RWY 21R. I DO NOT RECALL RECEIVING A HOLD SHORT CLRNC; AND MY FO WAS POSITIVE THAT WE WERE CLRED TO CROSS RWY 21R AT TXWY F. THE PROB SEEMS TO BE A MISCOM BTWN US AND GND CTL. WE UNDERSTOOD THAT WE WERE CLRED TO CROSS RWY 21R. GETTING SEVERAL ACARS MESSAGES AT THIS TIME ALSO LED TO SOME CONFUSION. I RECALL ASKING HIM TO READ THE FIRST MESSAGE TITLED; 'YOU ARE ILLEGAL;' AS THIS ISN'T A NORMAL MESSAGE. HE WAS HEADS DOWN AS WE APCHED THE RWY READING THE DIFFERENT MESSAGES; AND RELAYING THE INFO TO ME. PART OF THE FIRST 'YOU ARE ILLEGAL' MESSAGE STATED THAT WE SHOULD REMAIN AT THE GATE. WE DIDN'T UNDERSTAND THIS; AS WE BLOCKED OUT OVER 5 MINS EARLIER. SUPPLEMENTAL INFO FROM ACN 617908: SUDDENLY WE BEGAN TO RECEIVE MULTIPLE ACARS MESSAGES. I WENT HEADS DOWN TO READ THE MESSAGES. I ALSO OBSERVED AN ACFT JUST ROTATING AND CLBING OUT FROM THE SAME RWY. THE DEPARTING ACT PASSED US AT AN ALT OF ABOUT 50 FT.