On January 8, 2004, about 1430 central standard time, a Beech E-90, N48TA, operated by Tidewater Aero, sustained substantial damage when the cabin door opened in-flight and impacted the empennage during climb out near Galesburg, Illinois. The non-scheduled domestic passenger flight was operating under 14 CFR Part 135. Visual metrological conditions prevailed at the time of the accident. An instrument flight rules (IFR) flight plan was on file. The airline transport pilot and his six passengers reported no injuries. The flight originated at the Galesburg Municipal Airport (GBG), near Galesburg, Illinois, about 1425, and was destined for Mountain Empire Airport, near Marion, Virginia. The accident report, signed by the pilot, stated: At 1415 CST N48TA was loaded with one pilot, [pilot's name], PIC [pilot-in-command], for Tidewater Aero, two Virginia Beach (VA BCH) police officers, and [four] prisoners, being returned to VA BCH. The PIC was among the first aboard. A VA BCH police officer closed the aircraft door, while the PIC was in the cockpit. The door was not verified to be closed properly by the PIC. At 1425 CST, the aircraft departed [runway] 21 at GBG and started a climbing left turn. At approximately 2000 [feet] MSL, the "Cabin Door" annunciator light came on. At that point the PIC turned the aircraft further left to return to GBG for landing. Power was reduced, and the climb stopped. At [approximately] 1430 CST, 2 [miles southeast] of GBG, the door separated from the aircraft. ... Witness sitting at the side facing seat-opposite from the door said he could hear loud "air" noise coming from the door before it separated. Aircraft was in a left turn when the door separated. Upon returning to GBG, ground crew brought a ladder to unload passengers without incident. That report stated that "no one on board or on the ground was injured" and it listed no mechanical malfunctions in reference to the flight.