Deputy assistant commissioner Neil Basu said: “We must all accept that there is a possibility we will never understand why he did this.
“That understanding may have died with him.”
Four people died and 50 were injured when the extremist drove a rented car into people before stabbing a police officer.
There is no evidence that anyone else accompanied Masood during the rampage in which he murdered four people and injured 50 others. But the timing and sequence of the radicalisation of Masood, born Adrian Elms, will, the law agencies believe, provide important pointers towards what unfolded.
Masood had come to the notice of MI5 and Scotland Yard’s Special Branch in the past for associating with known extremists, but, security officials insist, had not been involved in plotting terrorist acts.
There has been speculation that the radicalisation process began when he went to work as an English teacher in Saudi Arabia in 2005.