Detectives from the Metropolitan Police's Counter Terrorism Command have now confirmed that the terrorist attack carried out by Khalid Masood was over within 82 seconds.
Through detailed investigation the Met can confirm the attack started at 14:40:08 on Wednesday, 22 March, when the car that Masood was driving over Westminster Bridge first mounted the northbound pavement.
He continued towards Bridge Street along both the footpath and road until 14:40:38 before crashing into the perimeter fence of the Palace of Westminster.
At 14:40:59, the first 999 call was made to the Met reporting the incident.
Masood left the vehicle and was shot by a police firearms officer inside the Palace of Westminster boundary at 14:41:30.
Deputy Assistant Commissioner Neil Basu, who is the Senior National Coordinator for UK Counter Terrorism Policing, said:
"Our investigation continues at pace. I am grateful for the public support so far, but I am asking for more help.
"If you knew Khalid Masood and had spoken to him in the months, weeks or days leading up to this attack please come forward.
"We still believe that Masood acted alone on the day and there is no information or intelligence to suggest there are further attacks planned.
"Even if he acted alone in the preparation we need to establish with absolute clarity why he did these unspeakable acts to bring reassurance to Londoners, and to provide answers and closure for the families of those killed and the victims and survivors of this atrocity.
"We must all accept that there is a possibility we will never understand why he did this. That understanding may have died with him.
"Nevertheless, we are determined to understand if Masood was a lone actor inspired by terrorist propaganda or if others have encouraged, supported or directed him. If the latter proves to be the case, they will face justice.
"Whilst the attack lasted only 82 seconds it will remain in the memories of many forever. The thoughts of all those working on this investigation are with those so deeply affected.
"Thank you for the public support so far and the early media restraint.
"The response has been outstanding, and that includes the joint emergency services response. Hundreds of officers continue their hard work, and they are supported by colleagues across the country, and the whole UK policing family.
"The way that communities in London, and throughout the UK, have come together in the wake of this attack shows that we will not give into those who seek to breed discord and fear.
"Terrorists have tried to tear this City apart before and they have never succeeded. A very small minority of people have tried to use this as an opportunity to stir up hate, but once again we stand together in calm defiance. We will not allow them to divide us."
Anyone with any information is asked to contact the Anti-Terrorist hotline on 0800 789 321.