Young Muslims in the U.K. who refuse to shop at British retailer Marks and Spencer because it is mistakenly identified as Jewish-owned could be on the path toward radicalization, Britain’s most senior ranking Muslim police officer has warned.

Speaking with The Guardian, Scotland Yard commander Mak Chishty warned that Muslim children as young as five-years-old were already on the path toward the kind of Islamic extremism espoused by the Islamic State in Iraq and Syria (ISIS).

His concerns are fueled by an ongoing trend among certain radicalized British Muslims who travel to the Middle East to fight with Islamic State militants in their battle to reestablish an Islamic caliphate.

The inclination toward Islamic radicalization must be countered with diligent monitoring of young Muslims, especially over sudden negative attitudes toward behaviors forbidden by Islam such as drinking alcohol or wearing Western clothes, said Chishty.

He said that if Muslim children vocalize radicalized views, such as the belief that celebrating Christmas or other non-Mulsim holidays is “haram,” or sinful, parents ought to be more vigilant about finding out where such views were derived.

“We need now to be less precious about the private space,” Chishty told the Guardian, especially as the Islamic State uses social media and other Internet fora to reach out to possible recruits, such as a 16-year-old British girl currently being investigated by Scotland Yard for trying to join her sister with the Islamic State as a so-called “Jihadi Bride.”

The private space Chishty was referring to included “walking down the road, looking at a mobile … someone in a bedroom surfing the net … someone in a [water pipe] cafe talking about things.”

British estimates say some 700 individuals have departed the U.K. to join the Islamic State, and about half of those have returned to Britain.

According to The Associated Press, one Western estimate says that between 16,000 and 17,000 foreigners from 90 countries have flocked to Iraq and Syria to fight with the Islamic State. While the Middle East and Africa are the leading regions for foreign Islamic State fighters, more than 4,500 of those came from countries in Europe as well.

The leader of the Islamic State Abu Bakr Al-Baghdadi has appealed to foreigners to join the group not just as fighters, but also as professionals such as doctors, lawyers, engineers, administrators and scholars, according to AP.

On Tuesday, Fox News reported that federal investigators have been struggling to keep up with the “social media barrage” by domestic U.S. supporters of the Islamic State.

They warned that military installations and events could be targeted in the “near term.”