LONDON — President Trump criticized Britain's counterterrorism approach in several tweets on Friday morning, following a suspected terrorist attack in London that injured at least 22 people on a subway train.

Trump said authorities “must be proactive” and that attacks in Britain had been conducted by “sick and demented people who were in the sights of Scotland Yard,” using a synonym for London's Metropolitan Police. It is unclear whether Trump was tweeting about previous attackers, or about whoever was behind Friday's incident. British Prime Minister Theresa May later commented on those remarks by Trump, saying that it was not “helpful for anybody to speculate on ... an ongoing investigation.”

May’s former chief of staff, Nick Timothy, was more blunt in his criticism, writing: “True or not — and I'm sure he doesn't know — this is so unhelpful from leader of our ally and intelligence partner.”

On Twitter, the president went on to say Friday morning that “the internet is their main recruitment tool which we must cut off & use better,” referring to terrorists in general. Trump also reiterated his demands for a “larger, tougher and more specific” U.S. travel ban.

“It keeps going and going, and we have to be very smart and we have to be very, very tough — perhaps we’re not nearly tough enough,” Trump said during a brief appearance in the Rose Garden on Friday.

Some of Trump's criticism may be justified to a certain extent. After three years of attacks in Europe in which security services frequently struggled to explain how the perpetrators were able to avoid detection, Britain has recently been wrestling with a very different concern: At least three of the five attackers who struck Britain this year were known to law enforcement officials.

Similarly, hate preachers were long able to test the boundaries of Britain’s freedom-of-speech laws by radicalizing individuals across the country while committing no punishable crimes according to British law. Putting an end to its previous wait-and-see approach, Britain has recently cracked down on several hate preachers in a strategy change that appears to fit Trump's demand to deal with terrorists in a “tougher manner.” At least rhetorically, May had demanded tougher counterterrorism measures before the June election, too.

What many experts in Britain are likely to agree on, however, is that Trump's broader proposals would hardly make Britain any safer. In fact, British authorities already are much more proactive than officials in most other nations, and May has little leeway to expand those powers.

British authorities already have expansive legal options

“There is a general sense in the intelligence community that agencies already have an awful lot of legislation at hand,” said Raffaello Pantucci, director of International Security Studies at the Royal United Services Institute in London, before the Friday incident.

Expansive laws already allow authorities to prosecute or monitor suspects in ways that would be impossible in most other countries across Europe. The key challenges, however, are less rooted in a lack of legal options than in operational capacity.

There are too many suspects to arrest or keep track of

To monitor one suspect 24 hours a day requires 20 officers on average. Hence, intelligence agencies are unable to consistently monitor the 3,000 or so people in Britain who pose a direct potential terrorism threat, given that about half of the nation’s officers would be needed to do so.

Other estimates put the number of suspects who would have to be monitored even higher, at up to 25,000 Islamist extremists. Only about 500 of them are believed to be constantly observed.

In Britain, the inability to track all suspects has led to calls for a new priority list to identify the highest-risk individuals. Although the frequency of arrests has already increased since the beginning of the year, there have been demands to arrest even more suspects before they can turn to violence. Analysts are urging caution, however.

Cracking down on nonviolent extremists has not gone well in other countries

“Theresa May ... should be careful about cracking down on nonviolent extremists,” said Frank Foley, a war-studies professor at King’s College in London.

“It’s a strategy the French have tried with little success, as it has alienated communities and led to a situation in which community members are often unwilling to share crucial information with authorities,” said Foley.

Britain says that it has been able to prevent several attacks because Muslim community members alerted the authorities early on, partially because it has established schemes dedicated to improving relations between communities and authorities. Meanwhile, such relations are often deeply fraught in France, which has favored cracking down on suspects over including communities in counterterrorism.

The role of online propaganda on radicalization remains disputed

Despite Trump's comments suggesting that there has not been sufficient focus so far on Internet-related radicalization, Britain has already sought to counter extremist ideology for years by funding counter-messaging initiatives. Besides that, analysts doubt whether spending more on such schemes would have a significant impact.

“In the case of the most recent attacks in Britain, it wasn’t about the Internet. Many of those involved were radicalized through face-to-face interactions,” said Peter Neumann, director of the London-based International Center for the Study of Radicalization, before Friday's incident.

A recent study by German authorities found that propaganda or chats on the Internet played a smaller role in radicalizing individuals than face-to-face interactions, often facilitated through friends or social circles. Other studies, focusing on different countries in Europe, including Britain, have similarly found that the impact of online propaganda may not be as big as often assumed.

Refugees do not pose the main terrorism threat

Trump's decision to use Friday's attack to reiterate his demands for a more extensive travel ban provoked particularly strong reactions on social media. The United States already has some of the world's toughest application procedures for refugees, and in Britain, the vast majority of recent attackers were born or had grown up in Britain.

The bigger question — with an impact that extends far beyond Britain — is whether any new strategy could increase safety in a country that has long been considered a role model in preventing attacks.

John Wagner in Washington contributed to this report.

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