A person who knew one of the three London Bridge terrorists has told how they called an anti-terrorist hotline about the murderer, who is also said to have featured in Channel 4 documentary, The Jihadis Next Door.
A BBC reporter said they had spoken to two people who said they had "warned police about one of the perpetrators" involved in Saturday's killing spree that left seven dead and 48 injured.
The Sun quoted a friend of the supposed ringleader of the attack - who it named only as 'Abz' - as saying he called police about the 27-year-old's extremist views which were said to have been ignited by YouTube videos.
The source, who has not been named, said he contacted police in Barking, east London, after the Watford-born married father-of-two discussed Islamic State-inspired terror attacks.
He told BBC's Asian Network that the jihadi had become radicalised after watching clips of US hate preacher Ahmad Musa Jibril.
He said: "He used to listen to a lot of Musa Jibril. I have heard some of this stuff and it's very radical.
"I am surprised this stuff is still on YouTube and is easily accessible.
"I phoned the anti-terror hotline. I spoke to the gentleman. I told him about our conversation and why I think he was radicalised."
After confirming Abz was allowed to keep his passport and was not arrested, he added: "I did my bit, I know a lot of other people did their bit, but the authorities did not do their bit."
The BBC stressed in a report on the Today Programme on Monday that the claim could not be verified and needed to be considered in the context of the 22,000 calls the anti-terrorism hotline received last year alone.
Police given warnings about terrorists involved in UK attacks
However, the allegation echoes those made in the wake of two most recent terror attacks - the bombing of the Manchester Arena on May 22 - that resulted in 22 deaths - and the Westminster Bridge attack on March 22 that saw Khalid Masood ploughing into pedestrians, killing four, before stabbing PC Keith Palmer to death. Police knew about arena bomber Salman Abedi "up to a point" and Masood was also on their radar.
Whether authorities missed opportunities to prevent the Manchester bombing is now subject to two inquires by MI5, although police have since said that 22-year-old Abedi was not known to the Prevent anti-radicalisation programme.
Police are yet to name the three men involved in the terror attack which began with a van ploughing into pedestrians shortly after 10pm before three men armed with knives rampaged through bars and restaurants at nearby Borough Market, indiscriminately stabbing bystanders.
Scotland Yard said the attackers names will be released as "soon as it is operationally possible".
Attacker seen in Arsenal shirt was a former Transport for London worker
Meanwhile, the family of Abz have reportedly asked to be left alone to "grieve in peace".
The Sun quoted the killer's brother-in-law as saying his family did not know what was going on and requesting space to mourn.
The newspaper said Abz was the terrorist pictured lying dead in an Arsenal shirt outside the Wheatsheaf pub, after being killed by police. He was said to have grown up in Pakistan but was brought up in the UK.
It further claimed that Abz was a former Transport for London, Topshop and KFC worker and had appeared in The Jihadis Next Door, where he was seen trying to radicalise children in his local park.
The man had, according to reports, become increasingly radical in recent years and was well-known in his local community.
One mother told the newspaper she had confronted Abz for trying to brainwash her children with extremist religious views in a park.
According to The Times, Abz 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 Abz 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," Chigbo said, according to a report in the Daily Record.
"He would preach to young Muslims at the flat.
"Sometimes up to six people quite regularly."
Abz 'had a reputation for being a bit shady and taking drugs'
Abz, according to reports, had quit working at KFC about two years ago.
A friend told The Sun: "Back then he had a reputation for being a bit shady and taking drugs."
The source added: "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."
Erica Gasparri was further quoted in the Daily Record as saying she had also raised concerns about Abz. She said he used to sit in a park opposite Northbury Primary School in Barking with two other men and talk to local children.
The mum-of-three said Abz would offer the kids sweets and claim to be "teaching them about religion".
'He had no knowledge of religion'
Another local who knew the attacker said he was kicked out of a mosque last year for supporting IS.
MailOnline reported that Abz was thrown out of the Jabir Bin Zayd mosque in Barking after interrupting a sermon to say that voting in the general election was "un-Islamic".
The manager of the mosque, who did not want to give his name, told the website that Abz halted the iman who was advising the community to vote.
"He started saying that voting was un-Islamic and we shouldn't do it," the manager said.
"He got very angry. I called him aside and said, please calm down. He refused, so I removed him. Thank God he followed me.
"When we got outside the room, he said you don't have any authority over me, only God has authority because this is a house of God. I said that might be true, but I am in charge."
The manger further stated that the man, who had not been seen at the mosque since being banned, did not appear to be well versed in Islam.
"He had no special friends here. He would arrive, pray and then leave. He said hello to people but generally kept himself to himself.
"He seemed an uneducated person. He seemed to have no knowledge of religion."
The Metropolitan Police has not yet commented on what, if anything, they knew about the attackers before Saturday's attack.
Speaking on the Today Programme on Monday, Commissioner Cressida Dick, declined to comment, saying it was a "very fast moving investigation". She instead detailed the number of arrests, raids and the amount of forensic material police had gathered.
Dick did however, admit: "I know people will be concerned that there have been three horrendous attacks in the last nine weeks. It's worth pointing out also that we've foiled five attacks, and since 2013 we've foiled 18 attacks."
The Press Association on Monday said that one of the London Bridge attackers was found carrying an identity card issued in Ireland when he was shot.