MD80 DEPARTING FROM IAH REJECTED TKOF BECAUSE OF TFC XING RWY DOWNFIELD.

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HOLDING SHORT OF RWY 33R AT IAH ON TWR FREQ 127.3; WE WERE CLRED FOR TKOF WITH A 010 DEG HDG ASSIGNED. WE INITIATED A ROLLING TKOF AND SOON THEREAFTER WE SAW AN ACFT TAXI ACROSS THE RWY AHEAD OF US FROM W TO E ON TXWY WL. WE ABORTED AT APPROX 70 KIAS; INFORMED THE TWR; AND CLRED THE RWY. WE QUERIED TWR AS TO WHY THIS HAPPENED AND HE STATED HE DIDN'T KNOW. WE HEARD NO CLRNC FOR THIS ACFT TO CROSS OUR RWY. OBVIOUSLY; WE WERE QUITE SURPRISED AND AGGRAVATED. AFTER CLRING THE RWY; WE REFERRED TO OUR CHKLIST AND DETERMINED WE WERE GOOD TO GO AHEAD WITH OUR FLT. THE CAPT ONLY MILDLY USED THE BRAKES AND IDLE REVERSE. BRAKE TEMP NEVER ROSE ABOVE WHAT IT HAD BEEN TAXIING OUT (100-110 DEGS C). THE INFRINGING ACFT WAS WELL OFF OUR RWY; APPEARING TO EXPEDITE AFTER POSSIBLY SEEING US HEADING HIS WAY. BEFORE RECEIVING OUR SECOND TKOF CLRNC; WE AGAIN ASKED TWR IF HE KNEW WHAT HAPPENED. AGAIN; THE RESPONSE WAS THAT THEY DIDN'T KNOW AND WERE TRYING TO FIND OUT. WE PERFORMED OUR TKOF AND FLT UNEVENTFULLY. WE NEVER HEARD A SINGLE WORD FROM THE INFRINGING ACFT AND DOUBT THEY WERE EVEN ON OUR FREQ. IT WAS A SMALL TWIN ENG BIZJET AND APPEARED TO HAVE SOME ORANGE OR RED FLAGS HANGING; LEADING ME TO BELIEVE IT MAY HAVE BEEN A MAINT REPOS.