The moment 29-year-old British citizen Salih Khater drove his silver Ford Fiesta into the car-proof security barrier guarding Westminster's Houses of Parliament was captured on a rooftop camera.
Footage obtained exclusively by the BBC and shared on social media shows a van driving down the same road as Khater, pulling into the same lane moments before he rams the car into barriers, injuring three cyclists.
The van then drives in the direction of Khater's car, but gets stuck in traffic while the Ford Fiesta crashes into the barrier.
A spokesperson for London's Metropolitan Police told journalists the mysterious van "was not of interest" but speculation over why it was there behind the suspect's car had already started brewing on Twitter.
One theory, however, is that the van was being driven by police officers, meanwhile, Scotland Yard's Assistant Commissioner Neil Basu has said "There wasn't a police car in pursuit of the vehicle."
Former counterterrorism officer and detective David Videcette thinks otherwise and tweeted: "The consensus between those of us who are ex-Police, is that this is likely an unmarked police vehicle, who spotted the Fiesta acting strangely before it rammed the barrier."