Westminster attack: London police make new arrest

British police investigating last Wednesday's attack on parliament made a new arrest on Sunday as authorities try to piece together the assailant's motive.

A 30-year-old man was arrested in the central city of Birmingham on suspicion of preparation of terrorist acts, London's Metropolitan Police said in a statement.

A dozen people have been arrested since 52-year-old Briton Khalid Masood launched an assault on parliament on Wednesday, killing four people and leaving 50 injured before he was shot by police.

Nine people have been released without charge, while a 58-year-old man remains in custody and a 32-year-old woman has been released on bail.

British police admitted on Saturday they may never know the motive behind this week's attack on parliament, after releasing all but one of 11 people held over the assault.

Police have now said they believe Masood acted alone and have issued a fresh plea for any information that might explain why he did it.

"We must all accept that there is a possibility we will never understand why he did this. That understanding may have died with him," said senior counter-terrorism officer Neil Basu.

A new breakdown of the attack revealed that Masood, a Muslim convert with a violent criminal past, took just 82 seconds to wreak havoc.

His car mounted the pavement on Westminster Bridge before driving along the road and footpath and crashing into the fence of the Houses of Parliament.

Masood left the vehicle and was shot by police, but not before fatally stabbing an unarmed policeman, PC Keith Palmer, who was guarding a gate.

"Our investigation continues at pace. I am grateful for the public support so far, but I am asking for more help," Basu said in a statement.

"We still believe that Masood acted alone on the day and there is no information or intelligence to suggest there are further attacks planned.

"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, and to provide answers and closure for the families of those killed and the victims and survivors of this atrocity."

The Islamic State group claimed that one of its "soldiers" carried out the attack, the worst in Britain since the July 2005 bombings that left 52 people dead.

But police are still trying to establish Masood's motivation.

"We are determined to understand if Masood was a lone actor inspired by terrorist propaganda or if others have encouraged, supported or directed him," Basu said. 

In one of the most dramatic scenes of the attack, Foreign Office minister Tobias Ellwood was photographed desperately trying to resuscitate the stabbed policeman.

Ellwood, a former military officer who lost his brother in the 2002 Bali bombings, spoke out for the first time late Saturday, saying he was "heartbroken" he could not save Palmer's life.

Ellwood has been hailed as a hero for his actions, but he said: "I played only a small part that day, doing what I was taught to do."

Palmer's family said they were "overwhelmed by the love and support for our family, and most especially, the outpouring of love and respect for our Keith".

To those who tried to save him, they added: "There was nothing more you could have done. You did your best and we are just grateful he was not alone."

The attack on Westminster and the attacker's reported use of end-to-end encryption of message services has prompted a fierce debate over the role of giant technology firms and encrypted messaging.

On Sunday morning, British Home Secretary Amber Rudd told the BBC that end-to-end encryption of messages offered by services like WhatsApp are "completely unacceptable" and there should be no "secret place for terrorists to communicate".

Local media have reported that shortly before launching the Westminster attack, the attacker sent an encrypted message via WhatsApp.

"That is my view - it is completely unacceptable, there should be no place for terrorists to hide. We need to make sure organisations like WhatsApp, and there are plenty of others like that, don't provide a secret place for terrorists to communicate with each other," 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."

In an interview with The Sunday Times, British Foreign Secretary Boris Johnson said earlier that the failure of Google to remove radical websites was disgusting.

He used the interview to issue the toughest condemnation by a British government minister of the way technology giants cash in by running advertisements alongside extremist videos.

“I’m furious about it,” he said. “It’s disgusting. They need to stop just making money out of prurient violent material.”