B757-200. ATCT LCL CTLR ALLOWED ACFT LNDG WITHOUT INSURING THE ACFT AHEAD WAS CLR OF THE RWY.

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DURING LNDG ROLLOUT ON RWY 34L AT SEA; WE OBSERVED THAT AN ACR ACFT WHICH HAD LANDED AHEAD OF US HAD TURNED OFF THE RWY; BUT LOOKED UNUSUALLY CLOSE TO THE RWY. AS WE RAPIDLY APCHED; WE SAW THAT THE OTHER ACFT WAS HOLDING SHORT OF THE WRONG HOLD SHORT LINE -- HIS ENTIRE ACFT WAS; IN FACT; COMPLETELY ON THE RWY SIDE OF THE HOLD LINES. OUR R WINGTIP PASSED BY THE OTHER ACFT'S TAIL WITH WHAT WE FELT WAS MINIMUM CLRNC. WE SIMULTANEOUSLY NOTIFIED TWR THAT THEY SHOULD TELL THE ACFT TO MOVE UP AND CLR THE RWY. TWR DID NOT RESPOND; AND BY THE TIME WE CLRED THE RWY AND TURNED R ONTO THE TXWY; WE SAW THAT THE ACFT WAS BEGINNING TO MOVE FORWARD PAST THE HOLD LINES. WE CALLED TWR AGAIN AND NOTIFIED THEM THAT THE ACFT HAD NOT BEEN CLR OF THE RWY WHEN WE HAD PASSED BY. TWR ACKNOWLEDGED THIS XMISSION. WE TAXIED TO THE GATE WITHOUT FURTHER INCIDENT. BOTH THE CAPT AND I WERE EXTREMELY SURPRISED AT THE SUDDEN OCCURRENCE OF THIS EVENT -- WE DID NOT PERCEIVE THAT THERE WOULD BE A CLOSE CALL UNTIL THE VERY LAST SECONDS BEFORE WE PASSED THE ACFT. PERHAPS WE WERE SIMPLY NOT EXPECTING THIS KIND OF RWY INCURSION. THE TYPE WE NORMALLY HEAR ABOUT ARE ONES WHERE THE OFFENDING ACFT TAXIES ONTO A RWY IN FRONT OF ANOTHER ACFT; NOT ONES WHERE THE PREVIOUS ACFT FAILS TO CLR.