The teenage suspect in the London bombing at Parsons Green Tube station was reported to the counter-terrorism Prevent programme months before the attack, it has been claimed.
The 18-year-old Iraqi refugee, one of two men arrested over the 15 September incident, was also said to have been in frequent trouble with the police and authorities.
His foster parents, Penny and Ronald Jones, reportedly struggled to cope with his behaviour.
It became such a concern he was even referred to Prevent – the government's deradicalisation programme – through his local authority Surrey County Council, The Sun reported.
It is not clear if anything was done by government agencies before the teenager became a suspect in the planting of the bomb that injured 30 people.
An unnamed source told The Sun: "I understand he was flagged up to Prevent. There are bound to be questions over what they did."
A neighbour of his foster parents said police made repeated visits to the home in Sunbury-on-Thames in Surrey, including as recently as two weeks ago.
Stephen Griffiths, 28, who lives opposite the Joneses, said the "lovely couple" were visited numerous times over the span of a month.
"They started off as normally dressed cops, then moved up in the police ranks, wearing black uniforms in an undercover car," he told Press Association. "They used to speak to Penny and Ron on the doorstep, but the last couple of times they went in the house."
He added: "You need to question whether the house was under surveillance. I think counter-terror police visited a few weeks ago, and if so, why wasn't something done sooner?"
The revelations raise fresh questions over what police, the intelligence services and the local authority knew about the teenager in the weeks leading up to the attack.
A spokesman for Prevent did not comment on the claims, while Surrey County Council said simply that it aims to provide the "best possible care and support to children' in foster care."
The Met Police said the teenager had not been arrested "in the last couple of weeks".
Ian Harvey, leader of Spelthorne Council in Surrey, said: "Clearly there should now be an independent inquiry about the actions of all relevant agencies to ascertain exactly what has been going on."
The teenage suspect, who was detained in Dover on Saturday, is one of two to have been arrested over the Parsons Green attack.
A 21-year-old man, named as Yahyah Farroukh, who was also said to have links to the Joneses, was detained by counter-terror police on Saturday outside a chicken shop in Hounslow.
He is registered to another address being searched by police in Stanwell, Surrey.
Farroukh's Facebook page says he comes from Damascus in Syria, and that he attended West Thames College, West London, where he reportedly learned English.
He listed his place of work as party promoters Dope Diamond Entertainment and BSQ London.
Penny Jones, 71, and husband Ronald, 89, received an MBE from the Queen in 2010 for their services to children after fostering hundreds of youngsters, including refugees.
Friend Alison Griffiths said the couple, who are currently staying with friends, are "great pillars of the community", adding, "they do a job that not many people do."
Following both arrests the UK's terror threat level has been reduced to severe from critical, the highest level.
Home Secretary Amber Rudd said police were making "good progress" in the investigation and urged "everybody to continue to be vigilant but not alarmed".
Magistrates have granted permission for both suspects to be held by police for questioning until next week.