LONDON (CNN) — A helicopter crashed into a construction crane atop a new luxury residential building in thick London fog Wednesday, killing the pilot and another person and sparking a line of flame as it plunged to the ground.
Nine others were injured in the crash that took place at the height of the morning rush hour in Vauxhall, south of the River Thames in central London.
One was critical, but police said the others did not appear to be seriously hurt.
The crash did not appear to be terrorism-related, police said.
Vauxhall, not far from Westminster, is where a new U.S. Embassy is being built. The area, which is a mix of industrial, business and residential use, is also home to the headquarters of the UK intelligence service, MI6.
Neither of the two people killed have yet been named.
The pilot is thought to have been the only person on board the helicopter, which was on a scheduled commercial flight from Surrey, southwest of London, police said.
The second person killed "was in close proximity" to the helicopter, a London Ambulance Service spokeswoman said, but gave no more details.
Construction workers were at the site when the crash happened but were not hurt, she said.
An eyewitness said rescuers pulled three injured people from two burning cars.
Thick smoke and flames billowed into the sky. The bent crane dangled from the building top, prompting police to cordon off roads in the area and evacuate residents and office workers.
The St. George Wharf construction project includes the landmark St. George Wharf Tower, also known as the Vauxhall Tower. Its developers say it will be one of Europe's tallest residential towers.
"I was standing outside having a cigarette when I saw the helicopter flying super fast towards the crane," said Rezart Islami, a builder who was working across the street from the 52-story building. "The pilot appeared not to have seen the crane because the helicopter hit it and went bang."
Islami said the rotor blade snapped off, the copter spun to the ground and burst into flames.
"The top off the crane also came off in the impact," he said. "It all happened so fast it was unbelievable."
Whipps said he heard the sound of the helicopter rotors suddenly cut out.
He did not see the impact but as he looked though a window, he "suddenly saw this enormous fireball, black smoke, shoot up from the side of a building."
Christopher Jen was at Vauxhall Tube station on his way to his marketing job when he spotted smoke rising nearby.
"We could hear all the sirens and you could smell the smoke as the train went past the site," he said in a submission to CNN's iReport. "The station announcers did tell us that there was a helicopter crash at Vauxhall and the engineers had to go check the tracks before we could leave."
Reporters allowed through the cordons to the deadly crash scene say little can be seen other than a dark spot where the helicopter hit the ground.
Many helicopters follow the Thames as a flight route through the city. This helicopter appeared to be flying further south than usual, Whipps said.
No cause has yet been given for the crash but visibility was poor in London on Wednesday morning, with fog and a very low cloud ceiling.
Chris Yates, an aviation security expert, told CNN it was likely there would be questions about whether the helicopter should have been flying in these conditions.
Under safety regulations, tall buildings must have navigation lights on top to make them apparent to low flying aircraft, he said.
Helicopters are not supposed to fly within 500 feet of tall structures in central London, he said.
The London Heliport is nearby on the banks of the Thames, at Battersea.
The incident caused major disruption for many people on their way to work.
Vauxhall, with a rail station, Underground station and large bus station is an important transit point for those traveling from the south to other parts of London.
The Vauxhall area has been earmarked as a major development site.