Minutes before the terrorist rampage in London on Wednesday, attacker Khalid Masood sent a WhatsApp message to an unknown person, authorities said Sunday. The message's contents — and its intended recipient — can't be accessed by police because the popular messaging service encrypted them, a top British security official said.
Masood used WhatsApp, which is owned by Facebook, just minutes before the attack that left three pedestrians and one police officer dead and dozens more wounded, The Associated Press reported.
Police have arrested 12 people in the investigation, including a 30-year-old man who was detained in Birmingham on Sunday on suspicion of preparing terrorist acts, the BBC reported. Masood lived in Birmingham.
Nine people arrested after the attack have been freed without charges, while one person was released on bail, AP reported.
Appearing on BBC and Sky News on Sunday, Britain's Home Secretary, Amber Rudd, urged WhatsApp and other encrypted services to make their platforms accessible to intelligence services and police as they try to find out more about the attacks.
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"We need to make sure that organizations like WhatsApp — and there are plenty of others like that — don't provide a secret place for terrorists to communicate with each other," she said.
A WhatsApp spokeswoman said the company was "horrified at the attack" and was co-operating with the investigation, the BBC reported.
Masood drove a rented SUV into pedestrians on Westminster Bridge before smashing it into Parliament's gates and rushing onto the grounds, where he fatally stabbed a policeman and was shot by other officers. A detailed police reconstruction found that the entire attack lasted 82 seconds. Scotland Yard has said it believes Masood, 52, acted alone.
Police are trying to pinpoint his motive and identify any possible accomplices, making the WhatsApp message a potential clue to his state of mind and social media contacts.
Britain's Telegraph reported that cybersecurity experts were expected to employ their own hackers to access the message.
Rudd said she would be meeting technology firms this week, adding that encryption that conceals a terrorist's actions is "completely unacceptable — there should be no place for terrorists to hide."
The company, which has a billion users worldwide, has said protecting private communication is one of its "core beliefs."
Metropolitan Police Deputy Assistant Commissioner Neil Basu said it may never be possible to fully determine Masood's motives.
"That understanding may have died with him," Basu said Saturday night as police appealed for people who knew Masood or saw him to contact investigators. "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."
The Islamic State group, which is losing territory in Iraq and Syria but still has radical followers in other parts of the world, has claimed Masood was a "soldier" carrying out its wishes to attack Western countries.
Masood had convictions for violent crimes in the U.K. and spent time in prison. He also worked in Saudi Arabia teaching English for two years and traveled there again in 2015 on a visa designed for religious pilgrimages.