What happened
Following the closure of the main cabin door by ramp personnel, the pilot observed a door ajar warning light on the captain's panel. The pilot noted that a similar situation had occurred during three of the previous four flight legs on the preceding day. To address the warning, the pilot pulled and reset the circuit breaker after 10 seconds, which extinguished the light.
The crew proceeded with takeoff. While climbing through 4,000 feet above the ground, a popping sound was heard, followed by a rapid decompression. The pilots observed that the main cabin door was ajar but did not attempt to inspect it for safety reasons. The crew reduced the aircraft speed to 130 knots indicated airspeed and landed the aircraft safely; the crew then noted that the door had separated from the aircraft.
Following the incident, search efforts located the door in a field along the flight path.