Khalid Masood, a 52-year-old with a string of convictions stretching back decades, was unmasked by police as the home-grown terrorist responsible for Wednesday's attack.
Eight people remain in custody after properties across the UK were raided, while a picture emerged of the lone-wolf killer's nomadic lifestyle.
He was born in Kent on Christmas day with the birth name Adrian Elms, before later converting to Islam, according to reports.
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 reportedly 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.
The death toll from his assault on the capital rose again on Thursday when a 75-year-old man became the fourth innocent victim to die.
The man, who has not been named, had his life support withdrawn at King's College Hospital.
Five people remain in a critical condition after Masood ploughed a car down Westminster Bridge and stormed the Parliamentary estate armed with two blades, fatally knifing Pc Keith Palmer.
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.
A spokesman said he was known by a number of different names and research into them was continuing.
After leaving Kent, it is thought he most recently spent time in the West Midlands, with a witness to an armed raid on a flat in Edgbaston saying: "The man from London lived here."
Masood is also thought to have spent periods living in London, Sussex and Luton.
Scotland Yard said he was not the subject of any current investigations before the massacre and there was "no prior intelligence about his intent to mount a terrorist attack".
But he was known to police and MI5 and had convictions for assaults, including GBH, possession of offensive weapons and public order offences.
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 40 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.
Three police officers were hurt, two of them seriously.
Police arrested three women and five men on suspicion of preparing terrorist acts after raids in London and Birmingham.
A house in Carmarthenshire, south west Wales, was also searched, Dyfed-Powys Police said.
A minute's silence was held nationwide on Thursday and crowds later gathered in Trafalgar Square for a candlelit vigil.
Theresa May said Masood was investigated some years ago in relation to concerns about violent extremism but was a "peripheral figure".
Home Secretary Amber Rudd defended the security and intelligence agencies, saying: "The fact that he was known to them doesn't mean that somebody has 24-hour cover."
As police and intelligence agencies mounted a massive investigation to piece together the killer's movements in the lead-up to the attack:
:: The Islamic State terror group claimed in a statement that the attacker was "a soldier of the Islamic State executing the operation in response to calls to target citizens of coalition nations".
:: Commentators pointed out the group had a record of opportunistically claiming attacks and it was significant the statement did not appear to claim it had directed the strike.
:: Carriage Gates, where Pc Palmer and Masood died, was back in use on Thursday evening, although armed police were at the entrance.
:: Roads around the Palace of Westminster, including Parliament Square, also reopened.