The helicopter pilot flying Kobe Bryant and seven others — including the basketball legend's daughter — sent an ominous message to air traffic controllers moments before the aircraft came crashing down.

Not long after they set off on a misty Sunday morning in Southern California, pilot Ara Zobayan told authorities he was going to climb to avoid a cloud layer.

Jennifer Homendy of the National Transportation Safety Board told reporters today the pilot then asked for air traffic controllers to provide "flight following" aide but was told the craft was too low.

About four minutes later, "the pilot advised they were climbing to avoid a cloud layer," she said. "When asked what the pilot planned to do, there was no reply."

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She said a radar suggested that the helicopter ascended to 700m and began a descending turn to the left.

She added that investigators are looking at weather conditions at the time of the crash, but they are also examining the possibility that other issues played a role in the crash.

"We take a broad look at everything around an investigation, around an accident," she said. "We look at man, machine and the environment, and weather is just a small portion of that."

THREE BODIES RECOVERED

Three bodies have been recovered at the site where basketball legend Kobe Bryant and his daughter died in a helicopter crash along with seven other people.

It is unclear at this stage who the bodies belong to, but authorities have described a "nightmare" situation on the ground in California as they try to recover everyone who was on-board.

Sheriff Alex Villlanueva told reporters: "It's a logistical nightmare in a sense, because the crash site itself is not easily accessible.

"They cannot access the crash site, it is a very rough terrain, dangerous even in daylight. We just want people to stay away."

He said the terrain was "rough" and that there was debris for 100 metres in every direction of the crash.

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Thick fog filled the air on the Sunday morning when Bryant and eight other people — including his daughter — set off on the helicopter ride in Orange County, California.

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They had been given special approval to take flight as the weather conditions were worse than usual standards for flying.

This meant they could proceed through the foggy airspace in Southern California.

But after circling for around 15 minutes, the helicopter plummeted more than 150 metres in just 15 seconds before smashing into a Los Angeles hillside — killing all nine on board.

Witnesses believe the helicopter dropped out of the sky so quickly that the nine victims likely "didn't suffer" as the chopper was destroyed instantaneously.

"My alarm bells went off because I thought, 'This is awfully low,'" sound engineer, Scott Daehlin told the New York Post, estimating it was "100, 150 feet" above him, yet invisible in the dense fog and low clouds.

Investigators work the scene of a helicopter crash that killed former NBA basketball player Kobe Bryant, his 13-year-old daughter, Gianna, and several others. Photo / AP
Investigators work the scene of a helicopter crash that killed former NBA basketball player Kobe Bryant, his 13-year-old daughter, Gianna, and several others. Photo / AP

The chopper finally flew off — and just "20 seconds later" he "heard the impact," describing it as a sudden "thump" as it crashed into a fog-covered hill.

"It was not very loud. You could hear the crushing, collapsing of fibreglass, Plexiglas," he said, adding the "rotors stopped literally immediately" without an explosion.

Now, the US National Transportation Safety Board (NTSB) has launched a "go team" of up to 18 investigators into the crash that killed Bryant amid questions about why the helicopter was flying low in foggy conditions.

Investigators will focus on bad weather and mechanical problems as a potential cause of the tragic crash that has been mourned by basketball fans around the world. Why the helicopter was flying in conditions so foggy that even LA police had grounded their choppers will also likely be raised.

Detailed analysis of the flightplan of Bryant's Sikorsky S-76B helicopter, built in 1991, showed it took off just after 9am from John Wayne Airport in Orange County and tracked north over Los Angeles before tacking left towards Thousand Oaks where Bryant's Mamba Sports Centre is based.

Audio from air traffic control reveals the experienced pilot, named as Ara Zobayan, was warned "you're too low" seconds before the helicopter disappeared from radar.

Kobe Bryant's helicopter crash. Photo / Supplied/news.com.au
Kobe Bryant's helicopter crash. Photo / Supplied/news.com.au

What exactly happened in the minutes leading up to the crash is unclear, however flightplan analysis published by New York Magazine shows the pilot was flying above hilly terrain and in thick fog, and using "visual flight rules" - or VFR - which means relying on sight rather than instruments to fly.

The chopper maintained what appears to be a steady pace for the first few minutes, travelling at an altitude of approximately 213m and speed of 241km per hour, a path posted by aircraft tracking website FlightAware.com shows.

But over Burbank, the craft's speed abruptly halved to about 120km per hour, and a reconstruction of the flight path shows pilot Ara Zobayan circled several times in an apparent bid to get his bearings, FlightAware shows.

New York City University aviation professor Paul Cline stressed while he had no direct knowledge of Bryant's incident, fog can quickly turn dangerous when pilots become disorientated.

When you get in the soup, your senses don't work," he said. "For me, I always feel like I'm falling to the right. Other people might feel like they're falling to the left, or climbing."

The helicopter eventually came down on a steep hillside in Calabasas, sparking a small bushfire.

Kobe Bryant and his daughter Gianna. Photo / Getty
Kobe Bryant and his daughter Gianna. Photo / Getty

SHERIFF'S HEARTFELT REQUEST

Police and firefighters were first on the scene and quickly established a containment area, according to LA sheriff Alex Villanueva, who has handed the fatal investigation to the NTSB.

Mr Villanueva said the NTSB was expected to arrive on the scene on Sunday night, but warned its "extensive investigation" would likely take "a great deal of time".

"It's a logistical nightmare in a sense because the crash site itself is not easily accessible," he said, adding a heartfelt request for those trying to get close to the site.

"However, we are now faced with wellwishers and people mourning who have descended on the area, on the residential community, and even the crash site itself.

"We have to reiterate it is off limits to everybody except the first responders and investigators."

Mr Villanueva warned all those attempting to access the site would be asked to present photo ID.