CRJ200 FLT CREW IS ADMONISHED FOR FORGETTING TO SHUT DOWN THE NUMBER TWO ENGINE BEFORE CLOSING UP THE ACFT AND DEPARTING.

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I HAD AN XA20 RPT TIME TO FLY A 4 LEG 11 HR DAY WITH CAPT. WE FLEW ZZZ TO ZZZ1 THEN BACK TO ZZZ; HAD A 2 HR BREAK THEN FLEW TO ZZZ2 AND BACK; ENDING IN ZZZ. ALL 4 LEGS WERE UNEVENTFUL. UPON FINISHING IN ZZZ; I GAVE THE CAPT THE FLT TIME; CALLED IN OUR TIMES AND FUEL BURN TO OPS; AND THEN GOT UP TO SAY GOODBYE TO THE PAX. ONCE THEY FINISHED DEPLANING; I GOT MY BAGS OFF THE AIRPLANE AND DID THE POSTFLT INSPECTION AND DID NOT NOTICE ANY VISIBLE DISCREPANCIES. AFTER THAT; MYSELF AND THE FLT ATTENDANT LEFT THE AIRPLANE. SHE HAD A FLT TO CATCH AND I WANTED TO BEAT TFC ON THE WAY HOME. THE CAPT SAID IT WAS FINE AND HE WOULD CLOSE UP THE AIRPLANE. THE NEXT DAY I GOT A CALL SAYING THE NEXT CREW ARRIVED AT THE AIRPLANE AND FOUND THE #2 ENG RUNNING AND THE PLANE CLOSED UP. I AM ALMOST POSITIVE I SAW HIM SECURE THE ENG BEFORE I LEFT THE COCKPIT. CONTRIBUTING FACTORS COULD HAVE BEEN LACK OF SLEEP PLUS A LONG DUTY DAY; PLUS THE CAPT STATED HE HADN'T FLOWN IN 5 WKS DUE TO VACATION AND RECURRENT GND SCHOOL; PLUS THE AMOUNT OF NOISE AT ZZZ COULD HAVE PREVENTED ME FROM HEARING OUR ENG AT THE IDLE SETTING DURING THE POSTFLT. I ALSO BELIEVE IT TO BE POSSIBLE THAT MAINT COULD HAVE BEEN ON BOARD FOR SOME REASON AFTER WE LEFT. IN THE FUTURE; I WILL PAY MORE ATTN TO THE CAPT'S PARKING FLOW AND CHKLIST; MAKING SURE I CONFIRM VISUALLY HE SECURES THE #2 ENG; PLUS I WILL PAY MORE ATTN DURING MY POSTFLT. SUPPLEMENTAL INFO FROM ACN 791000: I BELIEVED I PARKED AND SECURED OUR ACFT AT SPOT. I BELIEVE I FOLLOWED THE CHKLISTS AND MANUALS AND THE FO DID A POSTFLT INSPECTION WITH NO ABNORMALITIES. I RECEIVED A PHONE CALL FROM A SAFETY COMMITTEE MEMBER ASKING ABOUT FLT AND THE ENG SHUTDOWN. I SAID I WAS SURPRISED AND EMBARRASSED IF IT WERE MY MISTAKE. I'M 99% SURE I SHUT IT DOWN CORRECTLY. AND THE FO NEVER MENTIONED IT WAS RUNNING. WE ALL LEFT THE PLANE AND COMMISSARY WAS LOADING DRINKS. I CHKED OUT; WENT TO CRASH PAD. IF THIS WAS MY MISTAKE; I AM SORRY AND GLAD NO ONE WAS HURT OR INJURED. THERE ARE A LOT OF FRIENDS AND CO-WORKERS I CARE ABOUT NEAR OUR/MY ENGS. THIS IS A LESSON LEARNED FOR ME EITHER WAY. WE JUST WENT OVER CHKLISTS USAGE IN GND SCHOOL EARLIER THIS MONTH. AND I BRIEFED THE FO ABOUT DILIGENT USAGE AND THE CLASS. I WILL TAKE EXTRA MEASURES TO BE DILIGENT AND PATIENT WITH FLOWS AND READ/DO CHKLIST PROCS; DOUBLECHK AND EVEN TRIPLECHK.