Police stumped as all but one suspect freed in London attack

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

Armed police outside Westminster underground station in central London as police announce that they have made more "significant arrests" over terror attacks. Source: AAP

They have named 52-year-old Briton Khalid Masood as the man who killed four people in Westminster on Wednesday, but 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, 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."

A total of 11 people were held on suspicion of preparation of terrorist attacks in the wake of the attack.

But only a 58-year-old man arrested in Birmingham, the central English city where Masood last lived, remained in custody late Saturday.

One of the ten others who were released, a 32-year-old woman, is on bail -- the others have all been freed with no further action.

'Heartbroken' hero MP -

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