On March 9, 1999, about 1755 hours Pacific standard time, a McDonnell Douglas 600N, N626SB, entered instrument meteorological conditions (IMC) and collided with 4,700-foot mean sea level mountainous terrain about 7 miles north of Rialto, California. A company visual flight rules (VFR) flight plan was filed. The helicopter was substantially damaged. The commercial pilot, the flight officer (crewmember), and the passenger sustained minor injuries. The aircraft was operated by the San Bernardino County Sheriff's Department, San Bernardino, California, as a public-use aircraft under the provisions of 14 CFR Part 91. The local area flight originated from San Bernardino about 1708. In the pilot's completed report, and during an interview, he indicated that the accident occurred while on a routine law enforcement patrol flight. In summary, the pilot reported that he was on a mission to investigate circumstances related to an apparent abandoned vehicle. Upon arriving at the mountainside, he performed a toe-in landing to discharge the crewmember. While the crewmember evaluated the vehicle, the pilot orbited the area, and noted that low elevation clouds were approaching. However, the visibility was about 10 miles, and the clouds appeared to have been between 500 and 600 feet above the ground. After a few minutes, the crewmember returned to the helicopter, and they departed. Seconds after taking off the pilot lost visual reference with the ground. The pilot further indicated that he continued flying by reference to his flight instruments and the helicopter began spinning. Between 1 and 1.5 minutes later the spinning stopped, and the helicopter lost altitude until impacting the mountainside.