E120 CREW TOOK OFF WITHOUT THE FLT ATTENDANT BEING SEATED. THE COCKPIT DOOR WAS ALSO LEFT OPEN.

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THE CREW OF FLT XXX HAD COMPLETED ALL BEFORE TKOF CHKLISTS UP TO AND INCLUDING PAX BRIEFING; WHEN AT APPROX 120 YARDS BEFORE THE HOLDING POINT OF RWY 27 AT TLH; TWR ADVISED FLT XXX WAS CLRED FOR TKOF. THE CREW RESPONDED THAT THEY REQUIRED A FURTHER 30 SECONDS TO COMPLETE CHKLISTS. ATC REPEATED AGAIN THAT FLT XXX WAS CLRED FOR TKOF. THE FLT ATTENDANT WAS SLOWER THAN NORMAL COMPLETING HER CHKS; BEFORE COMING FORWARD TO GIVE HER CABIN RPT. AS THE ACFT APCHED THE HOLDING POINT SHE DID COME FORWARD AND PROCEEDED TO CLOSE THE CABIN DOOR WHICH THE CAPT (SELF) SAW INITIATED. AT THIS TIME; THE CAPT'S ATTN WAS DIRECTED TOWARDS ANOTHER ACFT ABOUT 3 MI OUT; CLRED TO LAND. HE HEARD THE FO COMPLETE THE CHKLIST AND CONTINUED SLOWLY PAST THE HOLDING POINT AND SLOWLY ONTO RWY 27 CTRLINE. THIS WAS DONE SLOWLY AND TOOK SOME 10-15 SECONDS TO ALLOW TIME FOR THE FLT ATTENDANT TO SIT AND STRAP IN. AN UNEVENTFUL TKOF WAS COMPLETED. TO THE BEST OF THE CAPT'S (SELF) KNOWLEDGE; THE CABIN DOORS WERE CLOSED DURING THE TIME FROM PASSING THE HOLDING POINT THROUGHOUT THE TKOF ROLL AND REMAINED CLOSED THROUGHOUT THE FLT. AT NO TIME DURING THE FLT DID THE FLT ATTENDANT OR FO MENTION ANY ABNORMALITY OR MENTION ANYTHING ABNORMAL ABOUT THE TLH DEP ALTHOUGH IT LATER TRANSPIRES THE CABIN DOORS WERE OPEN AND THE FLT ATTENDANT NOT STRAPPED IN FOR DEP. AT NO TIME WAS THE CAPT (SELF) MADE AWARE OF THE DOOR OR FLT ATTENDANT STATUS ALTHOUGH THE FO KNEW AND CONTINUED WITH HIS TKOF.