In a statement, Metropolitan Police deputy assistant commissioner Neil Basu said the public may never truly know why Masood committed the horrific act of terror.
He 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."
Masood’s grotesque lone wolf attack on the heart of London lasted only 82 seconds, but left chaos and suffering in its wake.
The vile terrorist acted alone when he brought chaos and turmoil to the streets of London
Mr Basu added: “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.
"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.
Masood’s grotesque lone wolf attack on the heart of London lasted only 82 seconds London terror attack victims Fri, March 24, 2017
ISIS have claimed responsibility for the London attack that killed four people
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"If the latter proves to be the case, they will face justice."
The family of PC Keith Palmer, who courageously gave his life to stop the crazed attacker, said while they “miss him so much” they are “incredibly proud” of him.
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They said the officer would be remembered for his "selfless bravery and loving nature".
The family of PC Keith Palmer released a heartfelt message last night thanking the paramedics and MP Tobias Ellwood who tried desperately to save the heroic policeman.
Masood acted alone when he killed four people and injured 50 others
PC Palmer's family released a message last night thanking the paramedics and Tobias Ellwood
The statement read: “There was nothing more you could have done.
"You did your best and we are just grateful he was not alone.”
In an earlier statement, Mr Ellwood said: "I am heartbroken that I could not do more for PC Keith Palmer who gave his life in holding the line against