The pilot of the helicopter that crashed and killed basketball superstar Kobe Bryant climbed to avoid a cloud layer just before slamming into a hillside, federal investigators say.

Jennifer Homendy of the National Transportation Safety Board said Monday the pilot received special clearance to fly in heavy fog, then minutes later advised air traffic controllers of the climb. The controllers received no further reply from the pilot.

Homendy said radar data indicates the helicopter proceeded to climb several hundred meters before taking a left turn and descending, then crashing into the hills north of Los Angeles.

At a briefing Monday, she described the debris field as "extensive." "A piece of the tail is down the hill, the fuselage is on the other side of that hill and the main rotor is about 91 meters beyond that," she said, adding that everything is looked at during the investigation -- the pilot, the aircraft, and the environment.

Federal rules do not require helicopters to carry black box flight data recorders like those used in commercial airliners.