The pilot of the helicopter that crashed into a hillside outside Los Angeles, killing former NBA superstar Kobe Bryant and eight others, told air traffic controllers in his last radio message that he was climbing to avoid a layer of cloud, an accident investigator said on Monday.

The pilot had asked for and received special clearance to fly in heavy fog just minutes before Sunday's crash and was flying at 1,400 feet (427 metres) when he went south and then west, said Jennifer Homendy of the National Transportation Safety Board, which went to the crash scene on Monday to collect evidence.

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The pilot then asked for air traffic controllers to provide "flight following" assistance, but was told the craft was too low, Homendy said.

About four minutes later, "the pilot advised they were climbing to avoid a cloud layer," she said.

"When ATC asked what the pilot planned to do, there was no reply. Radar data indicates the helicopter climbed to 2,300 feet (701 metres) and then began a left descending turn. Last radar contact was about 9.45am (17:45 GMT) and is consistent with the accident location.

Coroner's officials worked to recover victims' remains from the hillside outside Los Angeles where the helicopter crashed. Aviation experts say the accident might have been caused by the pilot becoming disoriented in the fog.

Some experts raised questions of whether the helicopter should have even been flying.

The weather was so foggy that the Los Angeles Police Department and the county sheriff's department had grounded their own choppers.

The Sikorsky S-76 crashed on Sunday morning, killing the retired athlete along with his 13-year-old daughter Gianna and everyone else on board and scattering debris over an area the size of a football field.

'Disoriented in the fog'

Crews recovered three bodies on Sunday and resumed the effort on Monday amid an outpouring of grief and shock around the world over the loss of the basketball great, who helped lead the Los Angeles Lakers to five NBA titles in a 20-year career.

The pilot was identified as Ara Zobayan. Several aviation experts said it was not uncommon for helicopter pilots to be given such permission, although some thought it unusual that it would be granted in skies as busy as those in Los Angeles.

But Kurt Deetz, who flew for Bryant dozens of times in the same helicopter, said permission is often granted in the area.

Fans of NBA basketball star Kobe Bryant pay their respects at a memorial outside the Staples Center at LA Live in Los Angeles on Monday [File: Mike Blake/Reuters]

The aircraft left Santa Ana in Orange County, south of Los Angeles, shortly after 9am, heading north and then west.

Bryant was believed to be headed for his youth sports academy in nearby Thousand Oaks, which was holding a basketball tournament on Sunday in which Bryant's daughter, known as Gigi, was playing.

Air traffic controllers noted poor visibility around Burbank to the north and Van Nuys to the northwest.

At one point, the controllers instructed the helicopter to circle because of other planes in the area before proceeding.

When it struck the ground, the helicopter was flying at about 184 miles per hour (299 km/h) and descending at a rate of more than 4,000 feet (1,219 metres) per minute, the data showed.

Randy Waldman, a helicopter flight instructor who teaches at the nearby Van Nuys airport, said it was likely the pilot got disoriented in the fog and the helicopter went into a fatal dive.

"It’s a common thing that happens in airplanes and helicopters with people flying with poor visibility," Waldman said.