A helicopter crashed into a crane on an unfinished apartment block in central London on Wednesday, killing the pilot and another person as it spread burning wreckage over the ground.
At least 13 people were injured after the aircraft broke off part of the crane attached to a tower of luxury flats at Vauxhall, near the headquarters of the MI6 foreign spy agency on the south bank of the River Thames.
The top of the 51-storey building, The Tower at One St George Wharf, was shrouded in mist at the time of the crash, which occurred during the morning rush hour at around 8:00 am (0800 GMT). Police have ruled out a terror attack.
Witnesses said the boom of the crane fell onto a street below, sending construction workers fleeing for their lives, while the helicopter plunged to the ground in a thick plume of black smoke before exploding into flames.
Police said the pilot, who was alone in the helicopter, was killed along with another person on the ground.
Commander Neil Basu of the Metropolitan Police said it was "something of a miracle" that there were not more fatalities.
Emergency services said six people were taken to hospital, five with minor injuries and one with a broken leg, while seven others were treated at the scene.
Charter company RotorMotion confirmed the dead pilot was Pete Barnes, an experienced instructor who had worked with the air ambulance and on several movies including the James Bond film "Die Another Day" and "Saving Private Ryan".
A spokesman said the 50-year-old, who also worked on the filming of last year's London Olympic Games, was "one of the most highly qualified helicopter pilots in the UK", adding that the company was "devastated" by his death.
Police later said they believed the second man was 39-year-old Matthew Wood from Sutton, south London.
London Fire Brigade, which deployed more than 50 firefighters to the blaze, said they rescued a man from a burning car. An AFP reporter later saw a charred vehicle lying alongside a pile of twisted metal from the helicopter, just metres from a major train line.
"There was a flash and the helicopter plunged to the ground. It exploded and you can imagine the smoke coming out of it," said eyewitness Paul Ferguson, who was working in an office near the tower.
The pilot set off from Redhill in Surrey, just south of London, bound for Elstree to the north of the city, but bad weather caused him to divert toward London Heliport, five kilometres (three miles) from the crash site, officials said.
The Air Accidents Investigation Branch is now looking into what happened, including suggestions that the crane did not have its aircraft warning light on.
Several construction workers said they had heard that the crane operator had been running late and was only halfway up to his cabin when the aircraft hit.
Witnesses described their terror as part of the crane crashed into the street below the tower, which is due for completion later this year.
Paul Robinson, 42, had been waiting to get onto the building site in his rubbish truck when he felt something hit his vehicle.
He thought the truck driver behind had driven into him. "Then I looked up, saw debris falling and ran like hell," he told AFP.
Looking back, "I saw the crane, the boom, hit the ground. It made a big bang."
The street was empty at the time, he said, but "if it was 30 seconds later, there'd have been cars there".
Local resident Nic Walker, 35, said he had been in bed when he heard a helicopter flying overhead, followed by "an almighty crash".
"I knew immediately what had happened -- it's extremely foggy this morning," he told AFP. "I put on some clothes and rushed outside to see if anyone needed help. There was burning fuel everywhere."
The crash caused gridlock on the city streets and temporarily shut down several train, bus and Underground stations in the area.
Prime Minister David Cameron paid tribute to the emergency services for their "rapid and professional response".