Seven of the 11 people arrested since the Westminster attack have been released with no further action, police have said.
The Metropolitan Police said two men remain in custody, while two women have been released on bail until late March.
Police are trying to establish whether attacker Khalid Masood acted alone.
They are also investigating whether he used the encrypted messaging service WhatsApp moments before driving across Westminster Bridge on Wednesday.
It has been reported that a message was sent from Masood’s phone two minutes before the attack.
Police said they have seized 2,700 items from 21 addresses, including “massive of amounts of computer data”.
Meanwhile, the Saudi Arabian government has confirmed that Masood worked in the country as an English teacher in two different spells between 2005 and 2009.
Four people were killed and 50 injured after Masood drove his car into pedestrians on the bridge, before stabbing unarmed PC Keith Palmer, 48, who was guarding Parliament.
Masood was shot dead by police.
Police said their investigation into the attack would focus on Masood’s “motivation, preparation and his associates”.
Those still in custody are:
A 58-year-old man from Birmingham
A 27-year-old man from Birmingham
A 32-year-old woman, who was arrested in Manchester, was released on bail until late March. A 39-year-old woman, from east London, was released on bail until late March.
On Friday, Metropolitan Police Assistant Deputy Commissioner Mark Rowley said officers would investigate whether Masood “acted totally alone inspired by terrorist propaganda, or if others have encouraged, supported or directed him”.
The Saudi Arabian embassy in London said Masood had been in the country from November 2005 to November 2006 and April 2008 to April 2009, when he worked as an English teacher.
In 2015, he obtained an Umra visa – allowing pilgrimage to Mecca – and was in the country from the 3 to 8 March.
The embassy said: “The Kingdom of Saudi Arabia strongly condemns Wednesday’s terrorist act in London, as it does all forms of terrorism.
“The attack in London this week has again demonstrated the importance of international efforts to confront and eradicate terrorism.”
PC Keith Palmer and Aysha Frade were among the victims
Two of Masood’s victims, Aysha Frade – a teacher in her 40s – and US tourist Kurt Cochran, 54, died on Wednesday.
Retired window cleaner Leslie Rhodes, 75, from Clapham, south London, died on Thursday evening when his life support machine was turned off.
Fifty people were injured in the attack, with 31 receiving hospital treatment. Two are in a critical condition, and one has life-threatening injuries.
Two officers remain in hospital with “very significant” injuries, one of whom has been identified as PC Kristofer Aves.
Prince Charles has visited some of the injured at King’s College Hospital, including Travis Frain
On Friday evening it was announced that Tobias Ellwood, the MP who battled to save the life of PC Palmer, and security minister Ben Wallace have been appointed to the Privy Council – the historic group that advises monarchs – in recognition of their responses to the attack.
The Metropolitan Police said Masood, 52, who had a number of previous criminal convictions, but none for terrorism, had used a number of aliases.
While he was registered in Dartford, Kent as Adrian Russell Elms at birth Masood was also known as Adrian Russell Ajao during his childhood.
Masood was shot after stabbing PC Palmer – both men were treated at the scene
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