On March 23, 1998, about 0740 hours Pacific standard time, a Bell 205A-1, N90230, owned and operated by the Los Angeles City Fire Department (LAFD), California, as a public-use aircraft, experienced an in-flight tail rotor system failure. During the attempted forced landing, the helicopter impacted trees and terrain approximately 1.5 miles northwest of its destination. The purpose of the flight was to provide air ambulance transportation for a seriously injured child. Visual meteorological conditions prevailed. The helicopter was destroyed, and the commercial certificated pilot and one crewmember were seriously injured. Three additional crewmembers and the passenger sustained fatal injuries. The local area flight originated from the Van Nuys Airport, California, about 0722. Upon dispatch, the helicopter proceeded about 8 miles northeast and landed about 0729 at the Stonehurst Elementary School playground, near the scene of an automobile traffic accident. The 11-year-old trauma patient (passenger) was loaded into the helicopter, and the pilot took off about 0731. Using the air traffic control call sign of "Lifeguard Fire Three," the pilot proceeded over Burbank in a south-southeasterly direction directly toward the Children's Hospital. About 0737, during cruise flight over Griffith Park, the pilot experienced a partial loss of control and transmitted that he was experiencing an emergency. About 1/2 minute later, the pilot indicated that he was making a forced landing. During the wreckage examination the helicopter's two tail rotor blades and the adjacent 90-degree gear box were observed missing from the tail boom. These components were subsequently located on a hillside debris path within approximately 0.1 mile of each other, and about 1 mile north of the main wreckage. The yoke assembly, which normally secures the rotor blades together and connects to the drive shaft, was observed fractured. A local metallurgical laboratory reported the yoke had a fatigue fracture. The investigation is continuing to determine pertinent facts and circumstances related to the event(s) which initiated the fracture.