Eyewitnesses say they saw Kobe Bryant's helicopter disappear into heavy fog shortly before it crashed today, killing the NBA legend, his daughter Gianna and seven others.

The exact cause of the tragedy remains unknown. America's Federal Aviation Authority and National Transportation Safety Board are investigating. But weather conditions around the crash site in Calabasas, California may have been a factor.

"There were low clouds and fog in the area at the time of the crash," said Weather Channel meteorologist Heather Tesch.

Multiple witnesses told TMZ they heard the helicopter's engine sputtering before it went down. And The Los Angeles Times has spoken to a witness who saw the helicopter flying unusually low, then vanishing into a cloud of fog.

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"It didn't sound right and it was real low. I saw it falling and spluttering. But it was hard to make out as it was so foggy," said Jerry Kocharian.

After the helicopter disappeared into the fog, he heard a "boom".

"There was a big fireball. No one could survive that," he said.

Tony Imbrenda, from the Los Angeles County Fire Department, said emergency services were first alerted to the crash by a group of mountain bikers, who spotted the aircraft "in distress" and found it smouldering on the hillside.

Bryant, 41, was flying aboard a Sikorsky S-76B helicopter, the same type he had used frequently in the past.

Flight records show it left John Wayne Airport in Orange County at 9.06am local time, and crashed shortly before 10am, just east of its intended destination, Thousand Oaks. Authorities received a 911 call at 9.47am.

The impact ignited a brushfire covering a quarter of an acre, making it more difficult for rescue teams to gain access.

"Our firefighters hiked to the site with medical equipment and hose lines to extinguish the fire, as it included the brushfire, debris from the helicopter, and the fire also included magnesium, which is very hard to extinguish because magnesium reacts with oxygen and water," fire chief Daryl Osby said at a press conference.

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Meanwhile, a helicopter overhead delivered paramedics to the scene and hoisted them down.

"They did a search of the area for survivors. Unfortunately, all of the passengers were determined to have perished," Mr Osby said.

LA County Sheriff Alex Villanueva said the flight manifest showed nine people were on board the helicopter, contradicting earlier reports that there were only five.

"There is wide speculation who the identities are, however it is entirely inappropriate right now to identify anyone by name until the coroner has made the identification and made notifications to the next of kin," he said.

The helicopter's flight path. It was bound for Thousand Oaks, where Bryant trained his daughter's basketball team. Photo / Supplied/news.com.au
The helicopter's flight path. It was bound for Thousand Oaks, where Bryant trained his daughter's basketball team. Photo / Supplied/news.com.au
Smoke rises from the charred remains of the helicopter. Photo / AP
Smoke rises from the charred remains of the helicopter. Photo / AP

Bryant's habit of using helicopters to avoid downtown Los Angeles' notoriously bad traffic was well known. During his playing career, he would charter a chopper to take him from his home in Orange County to Staples Center for Lakers home games.

A 2010 profile of Bryant, published in GQ Magazine, explained why he did it.

"It's a nice dash of glitz, a touch of showbiz that goes well with the Hollywood sign in the hazy distance. But sexy as it might seem, Bryant says the helicopter is just another tool for maintaining his body. It's no different than his weights or his whirlpool tubs or his custom-made Nikes," GQ wrote.

"Given his broken finger, his fragile knees, his sore back and achy feet, not to mention his chronic agita, Bryant can't sit in a car for two hours. The helicopter, therefore, ensures that he gets to Staples Center feeling fresh, that his body is warm and loose as mercury when he steps onto the court.

"If you make $23 million a year with your body, taking a helicopter to work is actually quite practical."

Bryant also used helicopters for more personal engagements. He once told ESPN he'd taken one to ensure he'd make it to his daughter's football game on time. On another occasion, he helped teammate Steve Blake get to a doctor's appointment.

This time he was reportedly taking Gianna, 13, to basketball practice.

The pair were on their way to the Mamba Sports Academy, located in Thousand Oaks, where Bryant coached and trained Gianna's Amateur Athletic Union basketball team.

Bryant himself opened the facility last year, and recently spoke proudly about the team's progress.

"It's just doing it piece by piece. It's been beautiful watching them grow," he told USA Today last week.

It was Gianna who got her father back into basketball following his retirement.

"Before Gigi got into basketball I hardly watched it, but now that she's into basketball, we watch every night," he told Showtime Basketball's All the Smoke podcast in January.

The pair were spotted together courtside at several Lakers games late last year, watching Bryant's old team play. A video clip of them discussing the action went viral.

Those were the first Lakers games Bryant had attended since his jersey was retired all the way back in December of 2017.

"It's not that I don't want to go, but I'd rather be giving BB (Bianca) a shower and sing Barney songs to her. I played 20 years and I missed those moments before," he previously told the Los Angeles Times.

"For me to make the trip up to Staples Center, that means I'm missing an opportunity to spend another night with my kids when I know how fast it goes.

"I want to make sure the days that I'm away from them are days that I absolutely have to be. I'd rather be with them than doing anything else."

Family kept him away from Staples Center - until, through Gianna, family brought him back.

"We just had so much fun because it was the first time I was seeing the game through her eyes," Bryant told the podcast.

"It wasn't me sitting there, you know, as an athlete or a player or something like that, and you know it's like, about me, and I don't like that. It was her, she was having such a good time."

Kobe and Gianna are survived by Bryant's wife Vanessa and their three other daughters, Natalia, Bianca and Capri.