On February 4, 2001, at 1324 eastern standard time, a Learjet 25B, N48WA, skidded off the runway at St. Lucie County International Airport, in Fort Pierce, Florida. The aero medical flight was operated by Able American Jets under the provisions of Title 14 CFR part 91, with a flight plan filed. Visual weather conditions prevailed at the time of the accident. The Learjet received substantial damage, and the pilot and crew were not injured. The flight originated from Ft. Pierce, Florida, at 1200. According to pilot, after taking off, he experienced a landing gear retraction problem. Unable to resolve this problem, the crew then decided to return to St. Lucie County International Airport for an emergency landing. Shortly after touch down, the left main landing gear broke away from the airframe. The pilot reported that he lost directional control, and the aircraft then skidded off to the left side of the runway. Examination of the aircraft maintenance records revealed that the landing gear assembly had been removed and reinstalled during a recent maintenance procedure. Further examination revealed that the left main landing gear trunnion pin was improperly installed and not in accordance with the Learjet maintenance manual instructions.