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 March.
Police are trying to establish whether attacker Khalid Masood acted alone.
Four people were killed and 50 injured after Masood drove his car into people on Westminster Bridge and stabbed an officer guarding Parliament.
PC Keith Palmer, 48, died of his wounds. 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 appealed for information from the public, and said officers would investigate whether Masood “acted totally alone inspired by terrorist propaganda, or if others have encouraged, supported or directed him”.
He added: “There might be people out there who did have concerns about Masood but did not feel comfortable for whatever reason in passing those concerns to us.”
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.
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 during his childhood.
In the early 2000s, he was convicted of causing grievous bodily harm after slashing a man across the face with a knife in a pub.
Masood was believed to have been living in the West Midlands before Wednesday’s attack, but had previously spent time in Luton, Crawley, Rye and Eastbourne.
He also worked as a teacher in Saudi Arabia.
The Royal Embassy of Saudi Arabia 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.
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