LONDON — Even as fellow European countries worry about hardened Islamic State fighters returning from Syria and Iraq, Britain has another problem: the re-emergence of a homegrown militant cell, Al Muhajiroun, one of Europe’s most prolific extremist networks, which was implicated in the London bombings of 2005.
After those attacks, the British government passed a raft of counterterrorism laws and embarked on a crackdown against Islamist extremists. Many were sentenced to prison or restricted to halfway houses for 10 years and sometimes more.
But on Monday, a co-founder of Al Muhajiroun, Anjem Choudary, was photographed near his East London home wearing a long white robe and a black electronic ankle tag. Government officials confirmed that Mr. Choudary, one of the country’s most notorious radical Islamist preachers, had been released from a probation hotel after serving more than half of a lengthy prison term for inciting support of the Islamic State.
He remains under close monitoring, but he has begun the gradual process of becoming a free man. And he is not the only one.
Having served their time, many members of Mr. Choudary’s old network are being released from detention. Far from chastened, they have begun to remobilize, vowing to take on far-right extremists and renew their decades-long campaign to eliminate democracy in Britain and establish a caliphate ruled by Shariah law, according to interviews with a handful of former members.
Founded in 1996, Al Muhajiroun, which has used various names over the years, has spent years effectively taunting the British security forces through its ability to continue operating despite being banned in 2006 for its links to terrorism. After a period of dormancy, the group is now remobilizing by continuing to change its name, adopting lower-profile tactics, using encrypted apps and meeting in secret locations.
According to former members, areas where the network is regrouping include East London; Luton, a town north of the British capital; and the surrounding county of Bedfordshire.
“Muslims are being attacked all over the world,” said Laith, a former member of Al Muhajiroun who would only give his middle name for fear of prosecution. “Our mission is much more urgent now, and with Anjem and the other brothers out of jail, it’s time to regroup and come out harder than before.”
For the British authorities, the possibility of the group’s re-emergence is a deep concern. This month, Andrew Parker, the director general of MI5, Britain’s domestic counterintelligence and security agency, issued a rare public warning about the continued risks of extremist networks and emphasized the threat posed by groups sympathetic to the Islamic State.
“In the U.K., there remain individuals who are inspired” by Islamic State propaganda, he wrote in the newspaper The Evening Standard, “despite having shown no interest in traveling to Syria.”
The security services have expressed particular alarm at the prospect of a newly energized Al Muhajiroun network trying to recruit members as some ISIS fighters and sympathizers attempt to return to Britain after the fall of the Islamic State’s so-called caliphate in Iraq and Syria. Britain has taken a tough line against potential returnees, canceling the passports of more than 150 people.
David Videcette, a British former counterterrorism detective, said British citizens who had joined the Islamic State would pose a clear risk if they returned.
“Suddenly, we are going to have trained fighters who perhaps have the motivation to carry out martyrdom attacks, exposed to a historical network of radicals,” he said.
For now, Mr. Choudary is prohibited from speaking in public or connecting with his old network. But several former members said that his release from prison, along with that of other figures, had emboldened the group, which was linked to 25 percent of all Islamist terrorism-related convictions in Britain between 1998 and 2015.
“Some activists have started to meet again and are testing the waters as they re-engage with their activism,” said Michael Kenney, an associate professor of international affairs at the University of Pittsburgh, who spent years embedded with the group as part of the research for his book, “The Islamic State in Britain.”
It is hard to overstate the role Mr. Choudary has played in motivating Islamic extremists. This month, for instance, the BBC reported that one of the attackers behind the Easter Sunday bombings in Sri Lanka, Abdul Lathief Jameel Mohamed, was radicalized by Mr. Choudary after attending his sermons during a one-year study abroad program in 2006.
“No other British citizen has had so much influence over so many terrorists as Choudary,” said Nick Lowles, the chief executive of a British anti-racist watchdog group, Hope Not Hate, which he says has identified 120 Islamist militants with links to the imam.
A representative of the British Home Office, which has responsibility for monitoring extremist groups, said that the government was aware of the potential threat posed by Mr. Choudary and Al Muhajiroun, and had rigorous procedures in place to handle it.
“When any terrorist offender is released from prison, we know about it and have robust covert and overt powers to investigate and manage any threat they may pose,” the government representative said. “Those released on license remain subject to close monitoring and strict conditions, which if breached can see them go back to prison.”
Most of those who were senior members of Al Muhajiroun said that they remained under strict surveillance, with their electronic communications monitored, which was confirmed by the Home Office. As a result, they say they carry out their activities discreetly and avoid any mention of or association with the group’s former name.
The activists say they have shifted their recruitment tactics from provocative public preaching and demonstrations to secret internet forums and smaller group meetings in inconspicuous locations. If the group uses a name that has not been identified as that of a terrorist outfit, then the gatherings are legal.
Most members of the group abide by what they call a “covenant of security” that prohibits attacks on non-Muslims in their country of residence. But it is a matter of individual choice.
In recent years, group members have become increasingly influenced by foreign radical groups like Al Qaeda and the Islamic State, inspiring them to carry out the attacks on and near London Bridge in 2017 and the murder of a British soldier, Lee Rigby, in the capital in 2013.
For many years, Mr. Choudary, a former lawyer whose preaching emphasized a jihadist ideology in which the West was accused of victimizing Muslims, managed to stay out of prison even as the authorities shut down many members of his broader network.
“Decrease and destruct, that was their tactic, and I have to admit it worked,” said Mohammed Shamsuddin, who was recruited to Al Muhajiroun by Mr. Choudary more than 20 years ago.
Sipping on a latte at a juice bar in a town in northwestern England, Mr. Shamsuddin said that the authorities could no longer crack down in the same way.
“Now, all the guys you arrested 10 years ago are being released into the open,” he said, “and they are healthier and stronger than before, and it’s come at a time when the police are stretched and Brexit is destroying the country.”
Many of the senior activists of the network who are now regrouping were monitored under the government’s Terrorism Prevention and Investigation Measures Act of 2011. That legislation provides the framework for a program that restricts the movements of individuals suspected of involvement in terrorism-related activities, through the use of electronic tags and overnight house arrest.
Some, however, said that being placed under the program’s restrictions had been more like a long vacation.
“It was amazing,” said a man who was subjected to the restrictions, flashing a picture of a brick property with a garden. “I was placed in a four-bedroom house by myself in the nicest part of Ipswich,” a town in eastern England.
“If anything, I got a good rest after years of hard work, got my energy back,” he said, speaking on the condition of anonymity because he was not legally allowed to talk about the program. “They spent all that money to achieve what in the end? I’m back on Twitter, back on Facebook, back with my brothers. I’m back in society, doing my thing.”
Some activists in the network denied that Mr. Choudary had encouraged acts of terrorism, claiming that he had directed their energy into an ideological struggle, not a violent one.
“The British government made a big mistake by putting Anjem in jail,” said Abdulla Muhid, a 42-year-old former member of Al Muhajiroun. “He believes in the covenant of security and was able to control the youth as they were getting their education from a learned person. Now, everyone is freelancing, getting radicalized through the internet.”
It is not clear how many activists belong to the network formerly known as Al Muhajiroun. Even during the group’s peak years in the late 1990s to early 2000s, it never had more than 200 dedicated members, according to Mr. Kenney of the University of Pittsburgh.
It is also unclear whether Mr. Choudary will reassert his leadership when his license conditions end in 2021. There have been group members who have challenged his leadership in the past, and some of those who are still active in the network suggested that he had been replaced. Still, many said that they were eagerly awaiting his return.
“People are waiting for Anjem to come out; they are waiting for that spark,” the activist who recently emerged from the restriction program said.
“These monitoring programs will do one of two things to you,” he added. “Either they will break you or make you hard-core. For me, I’m now rested and feeling more hard-core. I’m ready.”