Erelle returned to France. "I deactivated my avatar’s virtual existence, keeping only her Skype profile. Mélodie sent a final message, apologising, so that her sudden disappearance wouldn’t arouse suspicion," she writes.

"... A week later, the magazine sent my article to press, under a pseudonym. For me, though, that was only the beginning. The authorities, fearing the terrorists could trace my address and my identity, have twice asked me to change my phone number. I don’t live in my apartment any more. For my safety, I can no longer report on Isis and its networks. Drastic safety measures have been implemented at my workplaces."

"The authorities asked me to keep Mélodie’s Skype account open for ongoing investigations, and to keep an eye on threats toward me. I don’t check it very often. Sometimes, when I do, I’m greeted by terrifying messages. They started when someone claiming to be Bilel’s wife began sending intimidating monologues filled with insults.

"Recently, a journalist friend called to tell me he’d learned from a reliable source that there was a fatwa against me. I spent hours searching the web. After a while, I found a video about me. It shows me wearing Mélodie’s veil on my couch. There’s no audio, but it does include cartoon characters of a devil and French and Arabic subtitles. I’ve seen the video only once, but I remember every word. I don’t think I’ll ever watch it again."

Despite all this, Erelle says she has no regrets in taking part in the investigation.

"I did my job. I don’t know if I did it well, but I did it with a lot of precautions," she says. "I’m not afraid of yesterday. I’m not afraid today. But I’m very concerned about the future. In the way of thinking of these people, they never let go. When they call for revenge, they actually want to do it and they will try to do it even if they have to die to achieve it."

Photos: Rex Features

Journalist poses undercover as jihadi girlfriend to lure Isis fighter on Facebook