About 3 minutes into the positioning flight, at 3,000 feet, the flight crew heard a “loud bang” and became aware that the main passenger door had separated from the airplane. The pilot declared an emergency, diverted to a nearby airport, and landed without further incident. The separated passenger door was subsequently found on a nearby golf course. The co-pilot reported in a written statement that he closed the main passenger door and checked for a green light to ensure that it was secure. In a subsequent interview, he reported that he “believed” that the door warning lights were working, but he could not positively recall seeing the “Door Safe” light illuminated after closing the door. He also did not remember activating the “Recall” switch before takeoff. Ramp security video showed that the main passenger door appeared to close; it then re-opened momentarily and closed again before the airplane taxied out of the ramp area. The co-pilot reported that he re-closed the door because it did not secure on the first attempt. An examination of the recovered door and the door frame did not reveal evidence of a pre-existing mechanical malfunction or failure that would have precluded normal operation. The top half of the door was crushed from the impact with the ground, which prevented establishing the condition of door rigging before the accident. An examination of the fuselage revealed a skin puncture that matched the location of the door pull-out handle and is consistent with the door being in the open position when the puncture occurred. Although the door’s external handle was found in the closed position, smudging on the fuselage was consistent with the handle’s movement from the open to the closed position during impact with the fuselage. Although it is apparent from the cockpit voice recording that the flight crew referenced the pilot checklist at some stages during ground operations, crew challenge and response items were sporadically mentioned. After the sound of the passenger door closing, neither crewmember mentioned the door warning lights. At no point during ground operations through the takeoff sequence were the doors, door warning lights, or master caution system mentioned. The co-pilot mentioned the “Annunciator” check in the Before Takeoff checklist; however, no “Recall” response was noted on the recording. Activating the recall switch would have alerted the flight crew to an unsecured door before takeoff. Probable Cause: The failure of the co-pilot to properly close and latch the passenger door before departure, and the flight crew’s failure to ensure that the door was secure by using the door warning system. Contributing to the accident was the flight crew’s inadequate use of checklists.