German investigators will now be able to insert into users' cellphones and computers spy software (or a "Trojan horse") to access data in encrypted message services such as popular applications WhatsApp and Skype (AFP Photo/Lionel BONAVENTURE)

London (AFP) - The British government said Sunday that its security services must have access to encrypted messaging applications such as WhatsApp, as it revealed that the service was used by the man behind the parliament attack.

Khalid Masood, the 52-year-old Briton who killed four people in a rampage in Westminster on Wednesday before being shot dead, reportedly used the Facebook-owned service moments before the assault.

Home Secretary Amber Rudd told Sky News it was "completely unacceptable" that police and security services had not been able to crack the heavily encrypted service.

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"You can't have a situation where you have terrorists talking to each other -- where this terrorist sent a WhatsApp message -- and it can't be accessed," she said.

Police said Saturday that they still did not know why Masood, a Muslim convert with a violent criminal past, carried out the attack and that he probably acted alone, despite a claim of responsibility by the Islamic State group.

"There should be no place for terrorists to hide," Rudd said in a separate interview with the BBC.

"We need to make sure that organisations 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 end-to-end encryption was vital to cyber security, to ensure that business, banking and other transactions were safe -- but said it must also be accessible.

"It's not incompatible. You can have a system whereby they can build it so that we can have access to it when it is absolutely necessary," she told Sky News.

Rudd said she did not yet intend to force the industry's hand with new legislation, but would meet key players on Thursday to discuss this issue, as well as the "constant battle" against extremist videos posted online.

"The best people -- who understand the technology, who understand the necessary hashtags -- to stop this stuff even being put up, not just taking it down, are going to be them," she told the BBC.

- WhatsApp 'horrified' by attack -

WhatsApp said it was working with British authorities investigating the Westminster attack, but did not specify whether it would change its policy on encrypted messaging.

"We are horrified at the attack carried out in London earlier this week and are cooperating with law enforcement as they continue their investigations," a company spokeswoman told AFP.

US authorities last year fought a legal battle with tech giant Apple to get it to unlock a smartphone used by one of the shooters in a terror attack last year in San Bernardino, California.

The FBI's own experts ended up breaking into the device.

Social media giants are also coming under pressure over extremist content being posted on their sites.

Germany this month proposed imposing fines on social networks such as Facebook if they fail to remove illegal hate speech from their sites.

Google, meanwhile, has faced a boycott by companies whose adverts appeared alongside extremist content on its internet platforms, particularly its video-sharing site YouTube.