Eyewitnesses described how the attacker deliberately mounted the pavement with a car on Westminster Bridge before hitting more than 12 people - including three police officers returning from a commendation ceremony.
The driver then reportedly got out of his car at the Houses of Parliament where he attempted to force his way past a security check-point.
After being confronted by police he reportedly attacked an officer with a knife before he was shot a number of times.
Police say Masood was not the subject of any current investigations and there was no prior intelligence about his intentions to mount a terrorist attack.
Detectives say he was known to police and has a range of previous convictions for assaults, including GBH, possession of offensive weapons and public order offences.
The terrorist responsible for yesterday's attack sped across Westminster Bridge in a car, ploughing into pedestrians along the way, then ran through the gates of the nearby parliament building and stabbed a police officer to death.
A total of 29 people were also injured after a knife-wielding terrorist drove a car into pedestrians before he attacked a police officer outside the Houses of Parliament.
The Metropolitan Police today updated the number of those dead to four - including two members of the public, a police office protecting parliament and the attacker himself. Those killed included the attacker, Pc Keith Palmer, who he stabbed, and a woman in her mid 40s and a man in his mid 50s, who were mown down as the terrorist sped across Westminster Bridge in his car.