Westminster attack a 'wake-up call for tech giants over online terror content'

It came as a 58-year-old man arrested in Birmingham last week in connection with the incident was released without charge.

The man was detained on suspicion of preparing terrorist acts but has been told he faces no further action, Scotland Yard said.

British police are continuing to question a 30-year-old man; a woman has been released on bail, while 10 have now been released with no further action.

Facebook, Google and Twitter are among firms that have repeatedly faced calls to do more to detect and remove jihadist and other extreme videos and web pages.

Mr Mackey, the acting commissioner of the Metropolitan Police, was asked about the issue as he appeared at the London Assembly's Police and Crime Committee.

He stopped short of identifying any individual companies, but said: "I think these sorts of incidents and the others we've seen in Europe are probably a bit of a wake-up call for the industry in terms of trying to understand what it means to put your own house in order.

"If you are going to have ethical statements and talk about operating in an ethical way it actually has to mean something."

The debate flared up last week after it emerged that information on how to mount an attack was easily accessible in the wake of Khalid Masood's murderous rampage.

British Foreign Secretary Boris Johnson attacked firms over a "disgusting" failure to remove extremist material, while Home Secretary Amber Rudd will raise the matter at a meeting with technology bosses on Thursday.

Mr Mackey, who is leading Britain's largest force until new commissioner Cressida Dick takes up the post, also discussed the challenge posed by secure applications.

A furore over security services' access to terrorists' communications erupted following reports that Masood's phone connected with encrypted messaging service WhatsApp just before the attack last Wednesday.

Investigators are examining an "enormous" volume of digital information.

Mr Mackey said: "Some of that will be in secure applications. Some of that will be in a variety of formats that are more easy to analyse and work with.

"We work hard with the industry to highlight some of the challenges of these very secure applications.

"It's a challenge when you are dealing with companies that are global by their very nature because they don't always operate under the same legal framework as us."

WhatsApp has said it is "co-operating with law enforcement as they continue their investigations".

Mr Mackey said the method of causing "large-scale carnage" using "little more than a vehicle and a knife" will "naturally lead to consideration as to whether there is more we can sensibly do to prevent similar attacks".

He told the committee that, while detectives believe at this stage that Masood acted alone in his execution of the attack, the investigation to establish whether anyone else was involved in any way continues.