Image copyright PA Image caption The floral tributes outside Parliament have continued to grow

A man has been arrested in Birmingham by police investigating the Westminster terror attack, in which four people were killed.

The Metropolitan Police said the 30-year-old was held on Sunday on suspicion of preparing terrorist acts.

Another man, 58, arrested in Birmingham on Thursday, remains in custody.

Khalid Masood, 52, killed three people when he drove a car into pedestrians on Wednesday. He then fatally stabbed a policeman before police shot him dead.

The entire attack on Westminster Bridge and in the grounds of Parliament was over within 82 seconds.

The latest arrest is the 12th to be made by the Met Police in connection with the attack. Nine people have so far been released without charge.

A 32-year-old woman arrested in Manchester remains on police bail until late March.

On Saturday police said they believed Masood acted alone but they were also "determined" to find out whether he had been inspired by terrorist propaganda.

However, Scotland Yard said it was possible they would "never understand why he did this".

'No place to hide'

Revelations that Masood's phone may have connected with messaging app WhatsApp two minutes before he struck have prompted a debate about the responsibilities of messaging services.

Home Secretary Amber Rudd said encrypted messages must be accessible to intelligence services fighting terror, stressing there must be "no place for terrorists to hide".

She is holding talks with other EU ministers in Brussels to discuss ways of preventing further attacks and will later this week meet technology firms.

A WhatsApp spokeswoman said the company was "horrified at the attack" and was co-operating with the investigation.

Labour leader Jeremy Corbyn said authorities already had "huge powers", and a balance was needed between the "right to know" and "the right to privacy".

Media playback is unsupported on your device Media caption Former MOD security chief tells Today there could be politics at play in messaging app surveillance proposal

All messages sent on WhatsApp have end-to-end encryption, meaning messages are unreadable if they are intercepted by anyone, including the authorities and WhatsApp itself.

So while Masood's phone is believed to have connected with the app, police may not know what, if anything, was communicated.

Major-General Jonathan Shaw, formerly in charge of cyber security at the Ministry of Defence, said there was "a lot of politics at play" in the current debate because the tech companies were already cooperating with governments in other areas.

But he told BBC Radio 4's Today programme: "If you crack this nut, you simply move the case on to another level. Terrorists will use different methods - they will use other means of communicating. They will use different codes, hidden languages - private languages... the problem will mutate and move on."

Image copyright PA/Facebook Image caption PC Keith Palmer, Kurt Cochran and Aysha Frade (left to right) died in the attack

Those who died in the attack were: 48-year-old PC Keith Palmer; Aysha Frade, who worked at a college near Westminster Bridge; Kurt Cochran, who was on a trip from the US to celebrate 25 years of marriage; and 75-year-old retired window cleaner Leslie Rhodes.

In a tribute to them at Wembley Stadium on Sunday evening, four wreaths were laid on the pitch by Metropolitan Police Acting Commissioner Craig Mackey, London Mayor Sadiq Khan, FA chairman Greg Clarke and Culture Secretary Karen Bradley.

Fans and players at England's World Cup qualifier against Lithuania also observed a minute's silence before kick-off.

Image copyright PA Image caption Wreaths for the victims of the Westminster attack were laid in the centre of Wembley's pitch

Image copyright PA Image caption A group of women linked hands on Westminster Bridge on Sunday to pay tribute to the victims

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