Scotland Yard's top anti-terror officer, Mark Rowley, said the suspects were held overnight in the West Midlands and Manchester as he revealed that the killer's birth name was Adrian Russell Ajao.
The attacker, who was also known as Adrian Elms, was using the name Khalid Masood at the time of the atrocity.
Asked about where the 52-year-old Muslim convert had been radicalised, Mr Rowley said: "As I have said, our investigation focuses on understanding his motivation, preparation and associates.
"Whilst there is no evidence of further threats, you will understand our determination to find out if he either acted totally alone, inspired by terrorist propaganda; or, if others have encouraged, supported or directed him."
Asked whether Masood had travelled overseas, the officer said: "We are looking at his history."
But he reiterated Prime Minister Theresa May's comments in Parliament that, while he had been investigated previously, he has been a "peripheral figure" who was not implicated in any current probe.
Mr Rowley said investigators are keen to hear from anyone who knew Masood well, understands who his associates were or can provide information about places he has recently visited.
Searches at five addresses are continuing, while 16 have concluded.
Detectives have seized 2,700 items from the searches, including "massive amounts" of computer data, while around 3,500 witnesses have been spoken to.
Nine people remain in custody - including the two arrested overnight. The other suspects - three women and five men - were arrested in the West Midlands and London on Thursday and overnight on Wednesday. One of the women has been released on police bail.
A clearer picture of Masood's movements before he wrought carnage on Westminster is beginning to emerge.
He stayed in the Preston Park Hotel in Brighton, reportedly telling staff as he checked out that he was he was going to London. He added: "It isn't what it used to be."
The manager of the hotel said the terrorist had been "laughing and joking, telling us stories about where he lived" hours before the atrocity.
Meanwhile, the widow of a man attacked with a knife by Masood in a pub assault said her late husband was injured after defending someone during a row.
Masood was jailed for two years over the 2000 attack in the car park of a pub in Northiam, near Rye in East Sussex, according to a news report at the time.
The Brighton Argus reported that Masood, who stood trial as Adrian Elms, "left Piers Mott with a three-inch gash on his left cheek" requiring 20 stitches after an altercation with "racial overtones".
Adrian Baker, 51, who has lived in Northiam for 25 years, said Elms was known as a "troubled" character.
He added: "Piers was a really nice guy, an older chap who didn't deserve to get stabbed."
In other developments:
:: A fourth victim who died after being hurt in the attack was named as Leslie Rhodes, a retired window cleaner from Clapham, south London, who neighbours described as a "lovely man".
:: Mr Rowley hit out at "nonsense" from "armchair critics" when asked about Deputy Commissioner Craig Mackey's actions at Parliament, after it was revealed that the acting head of Scotland Yard was rushed away in the aftermath of the attack.
:: The counter-terrorism chief insisted Parliament's security arrangements were "proportionate" and had been developed over many years following claims from critics that the main entrance to the Palace of Westminster had long been a weak spot on the estate.
:: Police chiefs launched a major increase in the number of firearms officers on duty around the country.
:: Two people remain in hospital in a critical condition, one with life-threatening injuries. Two police officers hurt in the attack are also in hospital with "significant injuries".
Masood was known to police and MI5 and had convictions for assaults, including grievous bodily harm, possession of offensive weapons and public order offences.
Home Secretary Amber Rudd has defended the security and intelligence agencies, saying: "The fact that he was known to them doesn't mean that somebody has 24-hour cover."
Masood ploughed a hired car across Westminster Bridge and stormed the parliamentary estate armed with two blades, fatally knifing Pc Keith Palmer.
His victims on the bridge included an American tourist, and a "highly regarded and loved" member of college staff.
Kurt Cochran and his wife Melissa, from Utah, were on the last day of a trip celebrating their 25th wedding anniversary, and were visiting her parents, who are serving as Mormon missionaries in London. Mrs Cochran was badly injured.
Aysha Frade, who worked in administration at independent sixth-form school DLD College London, in Westminster, is understood to have been 43 and married with two daughters.