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The site of the helicopter crash that killed basketball star Kobe Bryant (inset). Image Credit: Reuters

Calabasas, California:  The helicopter carrying the basketball legend Kobe Bryant on Sunday morning circled over a golf course in Los Angeles' Griffith Park, awaiting clearance from air traffic controllers to continue its flight into the hills.

The weather 55 miles south in Orange County, where the helicopter had departed less than an hour earlier, had been fine - 4 miles visibility. Bryant had routinely made the same flight from his home on the coast to the Camarillo airport, near his basketball academy north of Los Angeles.

But now, up ahead, a fog so thick that it nearly blinded drivers on the freeway enveloped the hillsides near their destination. Visibility was so poor that the Los Angeles Police Department had grounded its fleet of helicopters. The pilot had a decision to make, one that might have proved fatal.

Turn around? Begin flying on instruments and head to a safe airport? The pilot, who by all accounts had a sterling safety record and was licensed to fly in inclement weather, kept going.

Sometime after its last contact with air traffic controllers at 9.45am, the aircraft slammed into a hillside at 1,085 feet.

On Monday, investigators were trying to figure out what went wrong, and they emphasised that no possibility, including a mechanical problem, had been ruled out.

"We take a broad look at everything around an investigation, around an accident," Jennifer Homendy, a member of the National Transportation Safety Board, said at a news conference Monday afternoon. "We look at man, machine and the environment, and weather is just a small portion of that."

Like many celebrities and business tycoons, Bryant had long moved around Southern California by helicopter to avoid the region's famous traffic.

On the day of the crash he was on his way to a special event, a basketball tournament he was coaching at the training camp he had co-founded in Calabasas.

In addition to Bryant and his daughter Gianna, 13, the crash also killed two of her teammates; an assistant coach; the helicopter's pilot; and three other adults, one of whom was a college baseball coach.