LONDON >> The boy’s teachers were growing increasingly concerned. He was speaking admiringly in school of Jihadi John, the notorious British executioner with the Islamic State, and expressing a desire to travel to Syria.

Twice, the teachers referred the boy — a teenager from Blackburn, in northern England — to a government program called Prevent set up to spot early signs of extremism and intervene before it was too late.

On both occasions, the boy — struggling with his studies after his parents separated and socially withdrawn because of a degenerative eye disease that blurred his vision — refused to participate in sessions intended to keep him from becoming radicalized.

The need for such programs has become all the more apparent in the wake of the Paris attacks in November, which were carried out primarily by European citizens who became radicalized at home. Britain’s program is viewed as something of a model by other European countries and the United States.

But encouraging fellow citizens to identify potential radicals has also raised questions about racial and religious profiling and the balance between security and civil liberties, igniting a debate here over whether Prevent holds the risk of further alienating Muslims in Britain.

At the same time, the British program has exposed the limitations of an approach that relies on voluntary cooperation from those who are identified as potential threats. In the case of the boy in Blackburn, whose name has not been publicly disclosed because of his age, the police later arrested him after they found that he had made a detailed plan for an Islamic State-inspired massacre in Australia.

Last October, he was sentenced for inciting terrorism overseas and became, at 15, the youngest person to get a life sentence in Britain in a terrorism case.

The Prevent program, started by Prime Minister Tony Blair in the aftermath of the July 7, 2005, London bombings, encourages and in some cases requires Britons to watch for signs of radicalization in their communities and to alert the authorities about people who could become risks, before they turn violent. Once someone is identified, the authorities judge whether the threat of radicalization is sufficient to justify further counseling; if so, the at-risk person is offered a place in a voluntary portion of the Prevent program known as Channel, which seeks to steer participants away from extremism.

Last year, Prime Minister David Cameron’s government expanded the program’s scope, making it a legal duty for schools, hospitals, local governments, social services and prisons to flag extremist behavior with the authorities. Opponents say that requirement risks turning Britain into a surveillance state where one section of the public is encouraged to snoop on everybody else.

Prevent’s 65 million-pound ($94 million) annual budget covers the cost of supporting community organizations that help at-risk groups, such as disaffected youth or British Muslim women isolated from mainstream society. Last month, Cameron announced a 20 million-pound fund to teach English to Muslim women. Communities that are cut off, he said, are potential breeding grounds for terrorists.

The Prevent program provides instruction to teachers, doctors, social workers, and prison and housing officials on how to detect early signs of radicalization. The program’s officers explain, for example, the influence of Anwar al-Awlaki, a U.S.-born cleric who was killed in an American drone strike in Yemen in 2011 but who continues to inspire young Islamic militants through videos of his lectures available online.

Teachers are supplied with dictionaries to help them identify Arabic words used by the Islamic State such as “Dawla/Dawlah,” a term used to describe the group by its supporters; the pejorative “kaffir/kuffar” to mean non-Muslims; and less obvious words like “rafidha,” a derogatory term for Shiite Muslims.

The Home Office said more than 400,000 public sector employees had attended lectures about the program. They are given a list of 22 “contributing factors” that signal an individual’s potential engagement with an extremist group, including a loss of interest in friends and activities, unwillingness to listen to other points of views, using derogatory names for another group and condoning violence.

Those indicators, described in a Prevent guidance booklet, also include people with “occupational skills that can enable acts of terrorism” such as civil engineering, pharmacology, or technical expertise, including knowledge of information technology, chemicals, or military training.

As part of the effort, schools have installed monitoring devices on school computers, enabling teachers and administrators to see what kind of material their students are viewing online.

However imperfect, Prevent is about “making teachers aware that some of their kids might go in the direction” of terrorism, said Raffaello Pantucci, director of international security studies at the Royal United Services Institute. He raised the example of four girls from the Bethnal Green Academy in London who traveled to Syria last year.

“Clearly a mini social movement was going on within that school,” he said. “Those teachers now know what to look for and how to tell authorities.”

Patsy Kane is the executive director of two all-girls high schools in Manchester. Most of the 2,500 pupils are Muslim, she said, and the schools have already dealt with sensitive issues, such as students being forced into marriage. Prevent, she said, “is an added duty, a moral duty.”

“You can’t take away the risk, and we can’t control who is influencing our students outside of school,” she said. “But what we can do is to find alternative narratives, like those given by Islamic State defectors, and try to chip away at the group’s missionary zeal.”

Anyone flagged by the program is screened several times by the police and local officials. If a formal referral is made, the person is encouraged to take part in Channel, the de-radicalization program. This often involves an imam who plays the role of counselor, psychotherapist and religious scholar.

Intervention is voluntary because referred individuals are in what the government calls a “pre-criminal” stage.

The teenager from Blackburn and his family, for example, refused to participate in Channel after a referral from Prevent in July 2013 and another in November 2014. Although he was within his rights to decline, he was nonetheless put into a special school for youngsters showing unruly behavior.

In court later, the schoolboy said that his exclusion from regular school further radicalized him. That culminated in threats he made against a teacher in early 2015 that he wanted to “stab him in the neck with his pencil and kill him like halal.” He told another: “You are on my beheading list.”

When the police arrested him over the threats, they uncovered the terrorist plot in Australia. His phone contained images of the Islamic State, a Twitter account with 24,000 followers, Internet searches for bomb recipes, and encrypted instructions to a militant to behead a victim and run over police officers. Pretending to be a much older man, the teenager advised the militant on using a knife with a handle “perfect for tearing through throat.”

In the first six months of 2015, a total of 3,228 referrals were made, according to the latest figures from the National Police Chiefs’ Council.

Two-thirds of the referrals came from schools, social services and health care practitioners. Less than 10 percent came from local communities, suggesting that Muslims, among others, view Prevent with suspicion. As a result of the referrals, 46 people were prevented from traveling to Syria and 225 “are being successfully dissuaded from undertaking extremist activity related to Syria,” the police chiefs’ council said. A majority of referrals were of Muslims.

But success on the individual level can come at a cost.

Shashank Joshi, a senior research fellow at the Royal United Services Institute, said he was concerned that even “peaceful, legitimate debate could be dragged into the clutches of a security state.” That could undermine the government’s efforts and drive true extremists deeper underground, he said.

Some Muslims say they have been unfairly singled out and that Prevent has increased distrust of the government.

The program is “very subjective, and it very much depends on which eyes are looking,” said Mohammed Khaliel, an independent adviser to the Metropolitan Police in Prevent-related matters.

“It’s disproportionately focused on Muslims when the government says it isn’t,” he said. Occasional news reports about children being interrogated over spelling mistakes (living in a “terrorist” instead of “terraced” house, in one recent case) has stoked further anger, he added.

The London-based Waltham Forest Council of Mosques, which represents 70,000 worshipers, said in a statement that it had “no confidence in Prevent” and called the program racist. It reacted angrily to a questionnaire distributed by the Waltham Forest Council last year that asked primary-school children about their beliefs. Children were asked to respond to statements including: “Religious books are to be understood word for word,” “I believe my religion is the only correct one,” and “I would do what a grown-up told me to do even if it seemed odd to me.”

In the case of the Blackburn schoolboy, his lawyer said he knew that what he had done was wrong, but the presiding judge said that the teenager had paid only “lip service” to attempts to reform him.

The boy is being put through a mandatory program in a youth detention center and may be released from custody in five years, but only if he is considered purged of Islamic State views.