Construction workers at the scene of the helicopter crash in Vauxhall have told The Huffington Post UK that helicopters regularly fly dangerously close to cranes and skyscrapers.
One of the men, who works on St George's Tower, where the crane which was hit by the chopper sits, told HuffPost UK he was surprised such an accident had not happened before.
"I see helicopters all the time, flying down the river, coming way too close to the cranes. I've seen it many times before."
His colleague concurred that he had also seen many helicopters come dangerously close to cranes. "It's unbelievable what they do."
But they also told HuffPost UK that they believe red warning lights were on the crane the morning of the crash, and that the tower itself also had a warning light.
Police have said they will investigate whether a warning light was functioning on at the top of the crane in Vauxhall, which was hit by a helicopter.
Speaking on condition of anonymity, one of the construction workers said he believes "the fog will have been the problem."
Civil aviation expert Chris Yates told HuffPost UK fog could have appeared "quite rapidly and caught the pilot unawares".
"It may well have disorientated the pilot and hence the reason why he apparently flew into this crane at the top of the building," he said, but added it was too early to tell the causes of the crash.
There has been much unconfirmed speculation on social media about the warning light on top of the crane. Political blogger Guido Fawkes' posted a tweet from October where a man questioned the police helicopter service as to why the crane had no warning light on.
James Harvard, who wrote the original tweet, later clarified that he had seen a light on the crane since October.
A spokesman for Berkeley Group, which owns St George, the development company for the building, said in a statement: "We can confirm a helicopter collided with a crane at St George Wharf at 8am this morning.
"Our thoughts at this time are with the friends and families of those killed in this tragic incident. Emergency services are on the scene and authorities are investigating the circumstances.
"We are offering our full support and assistance to the emergency services."
A Civil Aviation Authority spokesperson said: “Helicopter operations in central London are strictly controlled. Single engine helicopters are required to fly along designated routes, which have been selected to provide maximum safety by routing helicopters along the river Thames, avoiding flying over built up areas as much as possible.
"Twin engine helicopters can operate in wider areas, however all aircraft operating in central London are subject to air traffic control clearance.
“There are requirements for lighting on tall structures. In addition, where appropriate, very tall structures are also notified to pilots for flight planning purposes, as was the case with the crane that was involved in this morning’s accident.
“The independent Air Accidents Investigation Branch is now investigating today’s accident to establish the facts. The CAA will be providing any assistance required.”
The top of the tower was partially obscured by cloud on Wednesday morning. Two people have been confirmed dead after a helicopter crashed, one of them the pilot.
Debris landed along Wandsworth Road, on the corner of Miles Street, yards from the Flower Market.
There are strict rules governing helicopter flights in the capital and the Air Accidents Investigation Branch (AAIB) team will want to know if proper procedures were followed.