Two more "significant arrests" have been made in connection with the Westminster attack, police have said.
Scotland Yard's top anti-terror officer Mark Rowley said the suspects were held overnight in the West Midlands and north west as he revealed that the killer's birth name was Adrian Russell Ajao.
Mr Rowley appealed to the public for information about the killer, who was also known as Adrian Elms but was using the name Khalid Masood at the time of the attack.
The officer also revealed that the fourth member of the public injured in the attack, who died in hospital last night, was 75-year-old Leslie Rhodes from Streatham, south London.
Mr Rowley said two people remain in hospital in a critical condition, one with a life-threatening injuries. Two police officers injured in the attack are also in hospital with "significant injuries".
Mr Rowley said: "We remain keen to hear from anyone who knew Khalid Masood well, understands who his associates were and can provide us with information about places he has recently visited.
"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."
Asked about where Muslim convert Masood, 52, 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.
"To that end in our continuing investigation and ongoing covert activity we have made further significant arrests overnight - in the West Midlands and north west."
He said searches at five address were ongoing and 16 have finished.
Detectives have seized 2,700 items from the searches, including "massive amounts of computer data".
He said officers had spoken to around 3,500 witnesses - including 1,000 who were on Westminster Bridge and around 2,500 who were in Parliament.
Asked whether Masood had travelled overseas, Mr Rowley 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" not implicated in any current probe.
Mr Rowley reacted angrily when asked about Deputy Commissioner Craig Mackay's actions at Parliament, after it was revealed the acting head of Scotland Yard was rushed away in the aftermath of the attack.
He said: "It's frustrating that we get nonsense like this from armchair critics."
Mr Rowley said nine people remained in custody - including the two arrested overnight.
The other suspects - three women and five men - were arrested in the West Midlands and London yesterday and overnight on Wednesday. One of the women has been released on police bail.
As detectives appealed for more information about Masood, a picture of the middle-aged murderer of many aliases who wrought carnage on Westminster began to emerge.
Masood, who ploughed a car down Westminster Bridge and stormed the Parliamentary estate armed with two blades, fatally knifing Pc Keith Palmer, had a string of convictions stretching back decades.
Hours before carrying out his atrocity, The Sun said he stayed in the Preston Park Hotel in Brighton, telling staff as he checked out that he was he was going to London.
He added: "It isn't what it used to be."
Officers were seen scouring the hotel in the wake of the attack after a receipt for his stay was found in the hire car he later ploughed down pedestrians with, the paper said.
A member of staff at the hotel said on Thursday night: "We have been instructed not to talk."
Further details emerged about his violent history, which included an episode when he stabbed a man in the nose in the driveway of a nursing home in Eastbourne in 2003.
More candlelit vigils for the victims are scheduled on Friday in Birmingham and London.
Scotland Yard said Masood - who was shot dead by police - was born in Kent on Christmas Day in 1964.
After leaving Kent, it is thought he most recently spent time in the West Midlands, with a witness to an armed police raid on a flat in Edgbaston on Wednesday night saying: "The man from London lived here."
Masood is also thought to have spent periods living in London, Sussex and Luton.
He was known to police and MI5 and had convictions for assaults, including GBH, 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."
His victims on Westminster Bridge included a US tourist from Utah who was celebrating his wedding anniversary and a "highly regarded and loved" member of college staff.
Kurt Cochran and his wife Melissa, on the last day of a trip celebrating their 25th anniversary, 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.
Up to 50 other people were injured in the attack, with casualties including Britons, French children, Romanians, South Koreans, Greeks, and people from Germany, Poland, Ireland, China, Italy and the United States.