Metropolitan Police commander in London Mak Chishty has said that teenagers who unexpectedly stop drinking, socialising with friends or wearing western clothes could also be becoming extremists.
In an interview with the Guardian, Chishty said there was now a need for "a move into the private space" of Muslims to spot views that could show the beginning of radicalisation far earlier.
He said this could be shown by subtle changes in behaviour, such as shunning certain shops, citing the example of Marks & Spencer, which could be because the store is sometimes mistakenly perceived to be Jewish-owned.
"We need to now be less precious about the private space. This is not about us invading private thoughts, but acknowledging that it is in these private spaces where this [extremism] first germinates," he said.
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"The purpose of private-space intervention is to engage, explore, explain, educate or eradicate.
"Hate and extremism is not acceptable in our society, and if people cannot be educated, then hate and harmful extremism must be eradicated through all lawful means," Chisty added.
The propaganda of Islamic State (IS) was so powerful, the officer said, that he feared his own children might be vulnerable.
"I am not immunised. If I feel the need to be extra vigilant, then I think you need to feel the need to be extra vigilant," he told his fellow Muslim parents.
He added that more work was needed to understand why youngsters were attracted to IS, "Some are bored, overqualified, underemployed...It is not a holy war.