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The Leicester City owner's helicopter has crashed in a car park outside the club's ground as it left the stadium following a Premier League match.
Vichai Srivaddhanaprabha was in the helicopter when it came down at about 20:30 BST on Saturday, a source close to the family has told the BSports.
One witness said he saw Leicester player Kasper Schmeichel run out of the stadium towards the scene of the crash.
It is not known how many other people were on board the helicopter.
Leicester had drawn 1-1 against West Ham United at the King Power Stadium, with the match finishing about one hour before the helicopter took off from the pitch.
Witnesses said they saw it just clear the stadium before it spiralled out of control, with some describing seeing a fireball as it crashed.
The club said it was assisting police and emergency services and would issue a more detailed statement in due course.
Under Mr Srivaddhanaprabha's ownership, Leicester City won the Premier League in 2016, having started the season as 5,000/1 outsiders.
Freelance photographer Ryan Brown, who was covering the game, saw the helicopter clear the King Power Stadium before it crashed.
He told BSports Radio Leicester: "The engine stopped and I turned round and it made a bit of a whirring noise, like a grinding noise.
"The helicopter just went silent, I turned round and it was just spinning, out of control. And then there was a big bang and then [a] big fireball."
Sky Sports News reporter Rob Dorsett said the helicopter took off from the pitch, as it does after every game.
He said that after a few seconds it appeared to lose control and crashed into the car park just a few hundred metres away.
'I have seen staff in tears' - Reaction to Leicester helicopter crash
Image copyrightPAImage caption Thai billionaire Vichai Srivaddhanaprabha purchased Leicester City in 2010
Leo Bruka told 5 Live that he saw two police officers whose car was near the crash scene.
"One of them was looking for something and the other one, he ran straight away to the helicopter and he was trying to break the window," he said.
"Then there is an explosion and they pulled back because the fire was too hot."
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Media caption'It's horrible, horrible, horrible' - Leicester fans react to the crash
A Leicester season ticker holder, who did not wish to be named, said he saw goalkeeper Kasper Schmeichel run out from the stadium followed by "loads of security guards and stewards".
"I was stood outside the ground but it crashed on the other side," he said.
Another witness said it appeared "the tail propeller wasn't working, putting it into a spin".
Image copyrightPete White Image caption The helicopter shortly after taking off from inside the stadium
Andrew Brodie, assistant chief fire officer at Leicestershire Fire and Rescue Service, said the crash was "clearly serious and tragic".
He tweeted early on Sunday that he had just left "multiagency strategic meetings" at Leicestershire Police's HQ, and asked for people not to speculate on the causes or people involved.
"Think of families, friends, responders, and @LCFC and their fans," he said.
Leicestershire Police said officers were working alongside the other emergency services, the Air Accidents Investigation Branch (AAIB) and Leicester City FC to establish the circumstances of the collision.
At the scene
By Ian Stringer, BSports commentator
It is really eerie around Leicester at the minute.
I have seen staff, backroom staff and [the] first team in tears here this evening. The reception area is described as silent, people looking around and crying and not knowing what to do and consoling each other.
I remember the owners taking over. I said hello to them in native tongue, their face lit up. I have generated relationships with them, seen them in Thailand in Bangkok in their home surrounding, they are really nice people.
The chairman looked at me today on the way to the executive box and said "hello and good afternoon".
He donated millions of pounds to Leicester Royal Infirmary, and also gave money to the University of Leicester.
He's taken Leicester into his own heart - it's a hugely important place for him that he's made home. It's a one-club city here in Leicester, and we've welcomed him with open arms.
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Media captionWitness John Butcher: "The helicopter just span"
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Nick Rushton, leader of Leicestershire County Council, said he was "deeply concerned" by the crash, and paid tribute to emergency services staff for their "professional response".
Labour MP Jon Ashworth, who represents Leicester South, said he had been in touch with emergency services and thanked them for responding so quickly.
Leicester Mayor Peter Soulsby tweeted that his thoughts and prayers were "with all - particularly owners who've done so much for club and our city".
A match between Leicester City Women and Manchester United Women that was set to take place on Sunday has been postponed.
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In light and in respect of the recent events at King Power Stadium our @FAWC_ fixture against @ManUtdWomen has been postponed. A newly arranged date will be communicated once confirmed.
— Leicester City Women (@LCWFC_official) October 28, 2018
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End of Twitter post by @LCWFC_official
Former Leicester City and England striker Gary Lineker signed off on Match of the Day by saying it had been a "dreadful day", while BT Sport's Jake Humphrey - who was broadcasting from the King Power Stadium - said it was the "single hardest moment" of his career.
Skip Twitter post by @GaryLineker
That was the most difficult @BSportsMOTD Iβve ever hosted. Thoughts are with everyone at Leicester City. A terrible tragedy. Heartbreaking.
— Gary Lineker (@GaryLineker) October 27, 2018
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Our thoughts are with all at @LCFC following the major incident after their game at the King Power Stadium tonight.
— The FA (@FA) October 27, 2018
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Skip Twitter post by @mrjakehumphrey
In 21 years, delivering this news live, with little information was the single hardest moment. You can see the shock.
The BT Sport team remained professional, yet all our thoughts were, and remain with, everyone at @LCFC a genuinely special family club xxhttps://t.co/oOCWKmcQVo
— Jake Humphrey (@mrjakehumphrey) October 28, 2018
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End of Twitter post by @mrjakehumphrey
Several Leicester City players, including striker Jamie Vardy, and West Ham have tweeted since the crash.