As the full horror from Paris is revealed, the inevitable news has emerged that at least one of the gunmen had deep links and history as a radical in France (Raffaello Pantucci writes).

This came in the week that news emerged from Syria that an American drone had killed Mohammed Emwazi, a west Londoner who joined Isis and rose to become the group’s public executioner. The concept of the homegrown jihadist, the killer from within, only accentuates the horror of the terrorist act — something of which terrorist groups are fully aware and capitalise on to drive their brutal message home.

The concept of the homegrown jihadist is not new. For the most part they are drawn to extreme ideologies in their youth as they are exploring their individual identities and for one reason or another find salafi jihadism the most attractive.

It is a problem that has been faced in Europe for almost two decades. The problem is one that is not going away, and is becoming more complex as we see people drawn to these ideas but in some cases not ultimately acting on their terrorist impulse for years. In the case of both Omar Mostefai, the Bataclan murderer, and Emwazi, the revelation of their names uncovered news that they were people the authorities had long noticed as individuals attracted to radical ideas. For the public, this is almost as shocking as the news of the individuals’ western backgrounds. How is it possible that these young men could have been allowed to carry out their atrocities if they were known to authorities?

It is important to look at their individual histories to address the issue. For the Frenchman, the authorities had noticed as far back as 2010 that he appeared to be interested in radical ideas. However, they did not feel that he was part of what they identified as terrorist networks. He appears to have been a higher priority as a petty criminal.

While circulating in a milieu that was not dissimilar, Emwazi seems to have been drawn to radical ideas in his youth. Graduating from the University of Westminster in 2009, he tried to join al-Shabaab in Somalia, part of a larger community of west Londoners drawn to the internationalist jihadist message advanced by that group.

While these men chose a life of violence, what is not known is how many others with similar narratives drop off security services’ radar as they move on to peaceful lives.

For the authorities the problem is a thorny one. The maturation of an individual from young person interested in radical ideas to active terrorist-murderer is something that can take years (or in some cases an alarmingly short period of time) and is not necessarily something that happens in tidy isolation or with clear markers. The Charlie Hebdo killers were also of longstanding interest to the authorities, but had been relegated to secondary priorities, given their lack of overt terrorist activity. It was the same for the murderers of Lee Rigby in Woolwich. Emwazi and Mostefai are likely to be in this category too: background concerns who were superseded by more active plotters.

Unfortunately, ideologies are patient. And as long as these ideas continue to exist, the long tail of terrorist radicalisation will occasionally brutally lash our societies.

Raffaello Pantucci is director of international security studies at the Royal United Services Institute and author of We Love Death as You Love Life: Britain’s Suburban Terrorists