One of the terrorists who attacked London on Saturday, killing seven people and wounding dozens more, had been previously known to police and MI5, the Financial Times reported. Khuram Shazad Butt, 27, had been investigated in 2015, the same year he was filmed praying in front of an ISIS flag at Regent’s Park in 2015, according to the Financial Times.

Britain’s top counter-terrorism officer, assistant commissioner Mark Rowley admitted that Butt had been investigated in 2015 and that a member of the public had also called the anti-terrorism hotline to raise concerns. But Rowley said no evidence had been found of attack planning or criminality and the investigation dropped into the “lower echelons” of priorities for counter terrorism investigators, the FT reported.

The footage, eventually broadcast on Channel 4 in a documentary called "The Jihadist Next Door" on British jihadis, identified the Pakistani-born Butt as a zealous Islamist. The footage showed him in the company of leading figures from the now-banned al-Muhajeroune network.

Police named the second attacker as Rachid Redouane, 30, who claimed to be Moroccan and Libyan. He was not known to police and MI5 before the attack on Saturday.

Rowley said that he had seen "nothing to show that a poor decision has been made," according to the FT, adding that the second idividual was known to police.

The report raised concerns about police and intelligence agencies' abilities to monitor suspected radical Islamist cells in the UK. Also on Monday, the Telegraph reported that local police had been monitoring a cell of suspected terrorists in East London - that report followed the arrest on Sunday of 12 people, among them seven women – all of them in Barking and other parts of east London, where at least one of Saturday’s killers was believed to have lived. Those arrested were aged from 19 to 60.

So far, the perpetrators of all three terror attacks in the UK this year had been known to police, raising questions about police and intelligence agencies' abilities to keep the UK safe from terror attacks. That includes twenty-three year old Salman Abedi, the perpetrator of last week's attack at Manchester Arena, which killed at least 22 people attending a concert by American singer Ariana Grande. Khalid Masood, Westminster attacker, was in a pool of "former subjects of interest."

Before this weekend's attack, the Times of London reported that intelligence agencies had identified 23,000 potential jihadis living in Britain, of this 'pool' of potential terrorists, 3,000 are suspected of posing an “imminent threat” and are being investigated accordingly.

It's not clear whether the attackers who carried out this weekend's strikes are part of a wider network, the FT reported.