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The shadows of onlookers are cast on floral tributes to the victims of the March 22 terror attack pushed through the railings of the Houses of Parliament in central London on March 25, 2017.DANIEL LEAL-OLIVAS/AFP / Getty Images

Police in Britain say they may never know why Khalid Masood killed four people and injured 50 in last week's terrorist attack in London.

After four days of intense investigation that included arresting 12 people, raiding properties in six communities and obtaining information from more than 3,500 witnesses, police said on Saturday they were no closer to uncovering a motive for the attack. Of those arrested, nine have been released without being charged, two are in custody on suspicion of preparation of terrorist acts and one has been released on bail. Police have also said they believe Mr. Masood acted on his own.

"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 Neil Basu, the Deputy Assistant Commissioner of the Metropolitan Police who is also the ssenior national co-ordinator for U.K. 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."

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Mr. Basu said the investigation is continuing but there have been no further arrests or raids since Friday. He added that police "are determined to understand if Masood was a lone actor inspired by terrorist propaganda or if others have encouraged, supported or directed him."

After the assault, police said they believed Mr. Masood, 52, had been motivated by international terrorism, and Islamic State took credit for the attack. Prime Minister Theresa May also indicated that Mr. Masood, who was born in Britain, had been among a number of people known to the MI5 intelligence service several years ago as a potential threat. However, Ms. May said he had been a peripheral figure and he had not been under investigation recently.

Various media reports over the weekend have noted that Mr. Masood spent time in Saudi Arabia as an English teacher, but Saudi officials said he was not a target of their security service. Mr. Masood, who was born Adrian Russell Ajao and changed his name after converting to Islam, also had a long criminal record in Britain, dating back to 1983 for various offences including possession of a knife and assault.

Police have also released a detailed timeline of Mr. Masood's attack, which lasted just 82 second.

It began at 2:40:08 p.m. on Wednesday when Mr. Masood drove his rented SUV along Westminster Bridge and mounted the northbound sidewalk, hitting dozens of people. He continued along both the footpath and the road until 2:40:38, before crashing into the perimeter fence that runs alongside the Palace of Westminster, which houses Parliament. At 2:40:59, the first 911 call was made to police.

After hitting the fence, Mr. Masood left the vehicle and ran toward a set of gates leading into Parliament. He stabbed unarmed police officer Keith Palmer before being gunned down by a member of the security detail of a cabinet minister at 2:41:30.

Mr. Palmer's family released a statement on Saturday thanking those who tried to help him, which included Tobias Ellwood, a Member of Parliament who gave Mr. Palmer mouth-to-mouth and tried to stop the bleeding. "There was nothing more you could have done," the family said to all who helped. "You did your best and we are just grateful he was not alone."

On Sunday, Home Secretary Amber Rudd called on social media companies such as Google, Facebook and Twitter to do more to stop extremists from spreading their message. In an article in the Sunday Telegraph, Ms. Rudd said she had summoned executives from all three companies to a meeting about the issue. Those companies have come under fierce criticism among MPs for weeks over suggestions they have not done enough to block terrorists and other criminals.

"Each attack confirms again the role that the internet is playing in serving as a conduit, inciting and inspiring violence, and spreading extremist ideology of all kinds," she wrote in the paper.

"But we can't tackle it by ourselves … We need [social-media companies] to take a more proactive and leading role in tackling the terrorist abuse of their platforms."

Ms. Rudd also told the BBC on Sunday that messaging services such as WhatsApp should end the encryption of messages. Mr. Masood is believed to have sent an encrypted message on WhatsApp moment before the killing began.

"That is my view – it is completely unacceptable, there should be no place for terrorists to hide. We need to make sure organizations like Whatsapp, and there are plenty of others like that, don't provide a secret place for terrorists to communicate with each other," Ms. Rudd told the BBC's Andrew Marr show.

"We need to make sure that our intelligence services have the ability to get into situations like encrypted Whatsapp."