Masood killed three people and injured 50 when he drove a car into pedestrians on Wednesday. He then fatally stabbed a police officer before being shot dead by police – all within 82 seconds.
Police said it was possible “we will never understand why he did this”.
The family of PC Keith Palmer said his bravery would be remembered.
MP Tobias Ellwood, who was among those who tried to save PC Palmer’s life, has said he is “heartbroken” that his efforts were not successful.
Deputy Assistant Commissioner Neil Basu said: “From update 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.”
Writing in two Sunday newspapers, both the home secretary Amber Rudd and Foreign Secretary Boris Johnson called on social media companies like Google and Facebook to be “more proactive” in stamping out extremist websites.
