Officials said they discovered three separate parts of the chopper in a rugged area of the park about a mile north of the crash site. The Monday accident killed four people on board, including 11-year-old Norma Vides-Anaya of Sun Valley, a car accident victim who was being airlifted for emergency treatment.

  The aircraft parts--two tail rotor blades and a portion of the tail rotor gearbox--bolstered the theory that the tail began to fall apart as the helicopter approached its destination, Children's Hospital in Hollywood.

  The cause of what appeared to be a serious mechanical failure in the 22-year-old Bell 205A-1 chopper remained an open question.

"We have no indication as to what transpired during the flight that gave rise to the accident--yet," said National Transportation Safety Board investigator Wayne Pollack.

  Investigators said they hope to issue a preliminary report by next week.

  Fire Department officials, saying they had unsuccessfully asked the City Council to purchase new helicopters nearly every year since 1990, told reporters they are now hoping to buy a pair of new ones.

  As a dozen investigators combed the wreckage, the gravity of the crash began to sink in at fire stations across the San Fernando Valley--the workplaces of the three helicopter crew members who died in the crash-and at the Valley school where Norma Vides was a popular student.

  The 1,600 children who arrived on campus of Mount Gleason Middle School in Sunland were confronted by a fleet of media vans parked outside, and by a flag flying at half-staff.

  "I heard about the accident before I came to school, but I didn't think it was going to be Norma," said classmate Kathy Urrutia, 11, a close friend of Norma, her eyes downcast.

  She learned it was Norma when another classmate showed her the girl's photograph in a newspaper.

  "When you're young, you have dreams about going to college and getting a good job," Kathy said, her voice barely above a whisper. "When you die, all those things are over with."

 
  Crisis counselors were dispatched to the school and were in contact with the Fire Department's flight crews as well as with the city General Services Department mechanics who maintain the city's fleet.

  Les Iden, assistant general manager for fleet services for the city, said the 28 mechanics and supervisors in the helicopter maintenance unit were "second-guessing" themselves after the crash.

  "They're agonizing right not," Iden said.

  Pilot Steven L. Robinson, who survived the crash, underwent surgery Monday morning and was taken off a respirator, officials said. He suffered a skull fracture and brain contusions. Investigators have yet to interview him.

  The other survivor, flight crew member Dennis Silgen, was still on a respirator but was in stable condition.

  At Fire Station 90 in Van Nuys, firefighters spent Tuesday milling around the garage, talking, listening and consoling one another over the loss of their co-worker, Michael McComb, 48 a flight crew member killed in the crash.

  "Usually we can't wait to go out on a call, but right now, with all the mixed emotions, we're hoping we don't have to," said Tom Burroughs, who has been at the station 13 years.

  "A lot of us are taking it hard, a lot of us are in denial, a lot of us are angry," Burroughs said.

  Capt. Salvador Sedejas said the crew met informally in the morning with Mayor Richard Riordan and continued to talk after he left.

  The mayor offered his sympathies and fielded the crew's concerns about recent budget cuts in the department.

  Also killed in the crash were paramedics Michael A. Butler and Eric F. Reiner, both 33.

  Assistant Chief Dean Cathey said the department is hoping to buy two Bell 412 helicopters at a total cost of about $12 million, and is exploring the possibility of buying military surplus helicopters.

  Fire department officials said the Bell helicopter involved in Monday's accident had not received a tail rotor upgrade that the department's other Bell 205A-1 had received within the last five years.

  However, officials cautioned that it would be "a big jump" to conclude that the upgrade would have prevented Monday's crash.

  Federal investigators had seized the chopper's maintenance records, and city officials said they did not know if any parts in the tail rotor system had been repaired or replaced during the helicopter's most recent annual inspection Jan. 12. But they said the tail rotor also would been examined during a preflight check Monday.

  Mechanics examined the main and tail rotors on the department's other helicopters and tested samples from their fuel tanks before clearing the fleet for service Tuesday, Cathey said.

  Federal records show that the tail rotor assembly of Bell's 205A-1 had been the subject of seven "air-worthiness directives" -FAA-required repairs or modifications-since 1988. About half of all such directives issued in the last decade for that model were related to the tail rotor. However until the specific cause is determined, it will remain unclear weather the problems addressed by the directives were related to the crash.

  Gearbox problems in a mechanically similar chopper recently led the National Guard to issue special instructions to pilots of the HU-1 series helicopter, the military version of the 205A-1. The directive required pilots to fly at higher altitudes at night and to never stray far from sites suitable for emergency landings.

  In addition, FAA officials in 1999 began requiring a visual inspection of the tail rotor blade on the Bell UH-1H, on which the 205A-1 is modeled. FAA investigators found that certain Bell blades were prone to corrosion, and said the tail rotor should be examined every seven days.

  The NTSB's Pollack said investigators working on the probe included representatives from federal and local agencies and from Bell.