Terrorist Khalid Masood was "joking" with hotel staff in the run-up to his attack in Westminster - and used a phone messaging app minutes before striking.
Masood was described as "very friendly" by the manager of Brighton's Preston Park Hotel, where he stayed the night before the atrocity.
"He was laughing and joking," Sabeur Toumi told Sky News, adding that Masood was chatty with staff.
His comments were echoed by guest Michael Petersen, a businessman, who saw Masood in reception on the morning of Wednesday's rampage.
"The shocking realisation that you can stand next to somebody that is totally, perfectly normal," Mr Petersen said.
"Articulate, polite, presentable - and the guy is on his way to commit mass murder."
The pair spoke as Sky learned that Masood had used the WhatsApp phone messaging service at 2.37pm on Wednesday afternoon.
He launched his attack two minutes later, killing four people. It is unclear whether he was communicating with someone.
Police have released the first recent picture of Masood, taken within the last year.
They appealed for anyone who knew him to contact them and revealed there had been three new arrests, two which police described as "significant".
Two suspects, from Birmingham, remain in custody.
Six others arrested in Birmingham and a man arrested in Manchester were released on Friday evening, with no further police action.
A 32-year-old woman, arrested in Manchester, has been bailed pending further enquiries.
Fifty people were injured in the London outrage. Two remain critically ill and two police officers have serious injuries.
Scotland Yard's top anti-terror officer Mark Rowley revealed that the full birth name of the attacker - who used many aliases - was Adrian Russell Ajao.
It is understood Ajao, who was shot dead, changed his name to Khalid Masood after having a religious conversion.
Islamic State claimed Masood was one of its "soldiers" and "carried out the operation in answer to calls to target the people of coalition states".
According to a Sky source who met him in a professional capacity, he had a wife and three children.
The source described Masood as "a big bloke", adding: "He looked like he was a bodybuilder, (you) wouldn't want to mess with him."
He added that Masood was a "very religious, well-spoken man", who had "converted" to Islam.
"You couldn't go to his home in Birmingham on Friday because he would be at prayer," the source said.
Masood claimed in a CV to be an English teacher, though it appears he did not work in any of England's state schools.
Some reports suggested he had spent four years in Saudi Arabia, leading to speculation he may have been radicalised there.
Police said Masood had been "known by a number of aliases", including Adrian Elms.
While he was born in Kent on Christmas Day 1964, detectives believe he had been living most recently in the West Midlands.
His name is also registered as living at addresses in Brighton, Eastbourne, Luton and London.
An old black and white photograph shows Masood as a schoolboy footballer at Huntley Secondary School for Boys in Tunbridge Wells.
A neighbour at his most recent address in Winson Green, Birmingham, said Masood was a keen gardener who left three months ago.
In a statement, the Metropolitan Police said there was "no prior intelligence about his intent to mount a terrorist attack".
But he did have convictions for GBH, possession of offensive weapons and public order offences, spanning 20 years from 1983.
He was jailed for two years in 2000 for slashing a man's face with a knife in a row with "racial overtones" outside a Sussex pub.
Three years later, after leaving jail, Masood was accused of stabbing a man in the nose, leaving him needing cosmetic surgery.
He was jailed for another six months. He had not been convicted for any terrorism offences, police confirmed.
Earlier, Theresa May said Masood was a "peripheral" figure who had once been investigated by MI5 over concerns about violent extremism.
Anyone with any information about Masood can call the Anti-Terrorist Hotline 0800 789 321.