The Westminster terror attack carried out by Khalid Masood that left four people dead lasted just 82 seconds.
Police also said they believed Masood acted alone, and that his motivations may never be known.
The Metropolitan Police's Counter Terrorism Command have released an exact timeline of the attack that began at 14:40:08 when the car Masood was driving over Westminster bridge first mounted the pavement.
He continued to drive along the pavement for a further 30 seconds before crashing into the perimeter fence of the Palace of Westminster.
A spokesperson for the Senior National Coordinator for UK Counter Terrorism Police, said the effects of the attack would live with the victims and the wider world for far longer.
Deputy Assistant Commissioner Neil Basu said: "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."
Masood was shot by a police firearms officer at 14:41:30.
Mr Basu said the investigation into how and why Masood carried out the deadly attack continues, but warned "we may never understand why he did this".
"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.
- 14:40:08 - Masood drives vehicle onto northbound pavement of Westminster Bridge, shortly after bus stop D;
- 14:40:08 - 14:40:38 - pedestrians are injured on both the road and fence as he drives along Bridge Street;
- 14:40:38 - vehicle crashes into perimeter fence of Parliament;
- 14.40.51 - Masood leaves the crashed vehicle;
- 14:40:59 - first 999 call received by the Met;
- 14:41:30 - Masood shot by a police firearms officer.
"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.
"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."