The Metropolitan Police said two men, aged 27 and 58, both from Birmingham where attacker Khalid Masood was living, are still in custody for questioning under the Terrorism Act.
Of the 11 people held after the attack, a 32-year-old woman arrested in Manchester, and a 39-year-old woman arrested in east London, were released on bail until late March while seven had been set free with no further action.
A message was reportedly sent from Masood's phone two minutes before the attack.
Meanwhile, the Saudi Arabian government has confirmed that Masood was in Saudi Arabia three times and taught English there.
He taught English in Saudi Arabia from November 2005 to November 2006 and again from April 2008 to April 2009, a Saudi Embassy statement said.
Masood again returned for six days in March 2015 on a trip booked through an approved travel agent, the embassy said.
Masood drove his rented SUV into pedestrians on Westminster Bridge on Wednesday before crashing his car into railings and then running into the grounds of Parliament.
Armed with a knife, he stabbed a police constable, before being shot dead by police.
In the attack, four people were killed while 50 injured with 31 requiring hospital treatment. The Islamic State group claimed responsibility for the attack.
Masood was born as Adrian Russell Ajao in Dartford before converting to Islam. He is said to have used a number of aliases, including Adrian Elms, and was known to UK police and intelligences services.
His first conviction was in November 1983 for criminal damage and his last conviction was in December 2003, also involving a knife attack.
He was born to a white mother and black father and is said to have been affected by racism while growing up.
Scotland Yard has deployed bomb proof armoured vehicles on the streets of London following the attack.
"London, and the UK, are open for business, and we are out there in greater numbers to make sure that the public see a highly visible presence to help reassure them as they go about their daily lives.
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