The London Bridge terror attack ringleader appeared in a documentary about jihadis in the UK last year.
The 27-year-old from Barking, East London, appeared on Channel 4 and is believed to have been the leader of the trio who ploughed into pedestrians before stabbing people in pubs and restaurants in the area on Saturday night.
He is currently only being identified by his nickname Abz at the request of the Metropolitan Police.
In 2016, he appeared in the documentary alongside a notorious preacher as they prayed to an Islamic State flag in a London park.
Police continue to investigate the atrocity that left seven dead and 48 injured.
Officers made 12 arrests on Sunday as they raided several addresses including a tower block.
The brother-in-law of the attacker told The Sun: “We haven’t been told what’s going on.
We just want to grieve in peace.
It is understood the killer was a home-grown jihadi who came from Pakistan as a child but was brought up in the UK.
But in recent years he had become increasingly radical and was twice reported to anti-terror authorities, it has been claimed.
The revelations will put further pressure on Britain’s security services after Manchester bomber Salman Abedi also appeared to have slipped through the net before carrying out the atrocity.
Abz was well-known in his local community, with one mother claiming she confronted him for trying to brainwash her children with extremist religious views in a park.
A neighbour said he immediately noticed the Arsenal away shirt Abz was wearing after being shot dead by police outside the Wheatsheaf pub on Saturday night.
He said: “I looked on Twitter and saw one of the terrorists who had been shot by police and he looked 90 per cent like my neighbour – he was even wearing the same Arsenal shirt that I had seen him in at 5pm that evening.”
Officers unleashed a hail of bullets after the trio used a hired van to mow down pedestrians on one of the capital’s busiest bridges.
They then leapt from the vehicle and began knifing people at random in bars and restaurants.
According to The Times, Abz is the only one of the attackers positively identified so far.
He lived in a block of flats with his wife and two young children which was raided by police early on Sunday.
Ken Chigbo, 26, who lives in the same building, described him as “really sociable” and said the killer had invited him to a barbecue just last week.
“I know he was quite a devout Muslim, I heard him talking about the Koran,” said Mr Chigbo.
“He would preach to young Muslims at the flat.
“Sometimes up to six people quite regularly.”
An online CV suggests the attacker worked for Transport for London and took a course in teaching English as a foreign language.
He also worked at fast-food chain KFC but quit around two years ago, according to a friend.
One pal told The Sun: “Back then he had a reputation for being a bit shady and taking drugs.
“But all that changed when he became radicalised.
“He began stopping his neighbours in the street and asking them if they had been saying their prayers and when they had been to the mosque.”
One man who knew the killer told the BBC he had become so concerned by his extremist views he rang the anti-terrorist hotline.
He claimed the attacker was not arrested and was allowed to keep his passport.
Erica Gasparri, 42, said she had also raised concerns.
She said Abz would sit in a park opposite Northbury Primary School in Barking with two other men and talk to local children.
The mum-of-three said he would offer the kids sweets and claim edto be “teaching them about religion”.
Another local who knew the attacker said he was kicked out of a mosque last year for supporting ISIS.
The Met has not responded to claims the killer had been reported to them previously.
The terror gang left seven dead and 48 injured in central London before police marksmen cut them down.