Counterterrorism analysts had called for a crackdown on the group and its offshoots for years. Now, they wonder whether a tougher approach could have saved lives, and whether there are lessons for other countries with locally entrenched extremist groups.
It remains unclear under what circumstances the three attackers met, but researchers have linked Al-Muhajiroun to a number of successful and foiled British terrorist plots over the last two decades.
Despite being officially banned, authorities practically allowed the group and its nationally known co-founder Anjem Choudary to operate.
A frequent speaker at radical protests, Choudary tested the boundaries of Britain's freedom of speech laws, and encountered little resistance from authorities until 2015 when he was charged and later jailed for 5½ years for inviting support for Isis (Islamic State).
Choudary's arrest might have come too late to reverse the damage in Britain and across Europe. Being allowed to travel across the continent, the hate preacher had provided assistance and support to other newly founded extremist groups elsewhere, and extended his network in Britain.
"There had always been a debate between the police who wanted to arrest key figures, and intelligence services who wanted to keep extremist groups like Al-Muhajiroun intact, in order to monitor their members," said Peter Neumann, the director of the London-based International Centre for the Study of Radicalisation (ICSR).
"It's a balancing act which can go wrong." By allowing Choudary to continue his operations, the extremist cleric was able to radicalise individuals who perhaps would not have been attracted to extremism.
Some of them ended up joining Isis in Syria and Iraq. Others plotted attacks closer to home.
The criticism to not crack down on the group earlier has also targeted Prime Minister Theresa May, who was Home Secretary until last year, and for whom the latest terrorist bloodshed in London has now become a major factor in tonight's election. Polls indicate that May's Conservative Party continues to lose support, although it remains ahead of the Labour Party.
May attempted to seize back the initiative yesterday by suggesting she would change laws to fight the terrorist threat facing Britain.
"If human rights laws get in the way of doing these things, we will change those laws to make sure we can do them," May told the Sun newspaper. The paper also endorsed May's Conservative Party.
Her political opponents said she was trying to divert attention away from mounting criticism over the police funding cuts that happened during her six-year stint as Britain's Home Secretary.
May oversaw controversial budget cuts resulting in the elimination of about 20,000 police officer staff positions and prompting concerns over Britain's ability to prevent terrorist attacks.
There are now at least 3000 individuals who pose a potential terror threat, according to authorities, said one official who until recently worked in a counterterrorism position and wanted to remain anonymous because he was not authorized to speak. Thousands more individuals in Britain sympathise or support militant groups, he said.
To monitor one suspect day and night, an average of over 20 officers are needed, amounting to about half of the nation's officers if the Government wanted to put all high-risk suspects under surveillance.
The inability to monitor all suspects has resulted in several high-casualty attacks in the past. In the case of the July 7, 2015 bombings, authorities were initially monitoring some of the suspects before their attack but failed to notice early warning signs, a parliamentary committee concluded at the time.
More recently, the suicide bomber who struck in Manchester two weeks ago had also been reported to authorities by concerned residents, but was not put under surveillance.
"To make such a decision, authorities likely consider factors like associates of the suspects, foreign connections as well as connections with militant groups to determine whether to invest time and resources on an individual," explained Otso Iho, a senior analyst with the Jane's Terrorism & Insurgency Centre.
"But because more terrorists refrain from using illicit materials to prepare their attacks, their prevention has become much harder. Hence, authorities also need to look at possible networks and social circles in which those people are embedded and may be radicalised in," said Iho.
It is a logic which puts authorities in an uncomfortable position, given that the easiest way to monitor social circles of suspects would be to keep their networks intact. But as Sunday's attack has shown, it is a strategy which poses serious risks if individuals slip through the surveillance operations of authorities.
One of the attackers, 22-year-old Moroccan Italian Youssef Zaghba, was stopped by Italian authorities in March 2016 while on his way to Turkey to presumably join the Islamic State. Speaking to the BBC, his mother said that she told Zaghba following the incident: "You have to be perfect now. You can't look at strange things on the Internet or meet strange people."
Instead, he travelled to London, and disappeared from the radar of authorities - until last weekend.
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