The attack last week that killed five people, including the 52-year-old attacker, ended within 82 seconds, according to new revelations.
"We still believe that Masood acted alone on the day and there is no information or intelligence to suggest there are further attacks planned," said Met police Indian-origin Deputy Assistant Commissioner Neil Basu, who is the Senior National Coordinator for UK Counter Terrorism Policing.
"We must all accept that there is a possibility we will never understand why he did this. That understanding may have died with him," he said.
His remarks came after two of the 11 people arrested in connection with the attack were questioned by police to establish whether the attacker acted alone or had associates.
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Detectives from the Metropolitan Police's Counter Terrorism Command said that the attack started at 14:40:08 on Wednesday when the car that Masood was driving over Westminster Bridge first mounted the northbound pavement.
After a detailed investigation, it said he continued towards Bridge Street along both the footpath and road until 14:40:38 before crashing into the perimeter fence of the Palace of Westminster.
At 14:40:59, the first 999 call was made to the Met, reporting the incident, it said.
Masood left the vehicle and was shot by a police firearms officer inside the Palace of Westminster boundary at 14:41:30.
"Our investigation continues at pace. I am grateful for the public support so far, but I am asking for more help. If you knew Khalid Masood and had spoken to him in the months, weeks or days leading up to this attack please come forward," 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," he added.
Born as Adrian Russell Ajao in Dartford before converting to Islam, Masood was known to UK police and intelligences services. He was born to a white mother and black father and is said to have been affected by racism while growing up.