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.