When your office door is just metres away from your neighbours,  you don’t have much need for their landline: it’s easy to stroll across the hall for a chat, or send an email. But the staff of the Paris-based television production company Premieres Lignes were to come to regret not having their colleagues’ number on the morning of January 7 last year. As two gunmen entered the building and stumbled around looking for the offices of the satirical magazine Charlie Hebdo, alarmed Premieres Lignes staff locked their own office door, headed to the roof, and waited helplessly as the massacre unfolded below. Their continuing regrets over their lack  of heroic action is one of the most compelling sequences in a remarkable film airing tonight on BBC’s This World. Directed by five time BAFTA winner Dan Reed, Three Days of Terror: The Charlie Hebdo Attacks lays out in forensic detail the sequence of events that kicked off with the terrible massacre in the magazine’s meeting room.

Reed is no stranger to this territory, having similarly masterfully dissected terrorism attacks in Mumbai, Moscow and Nairobi. He is one of the most accomplished documentary makers working in Britain today (his recent masterclass at Sheffield Doc/Fest is well worth a listen). Docs on Screens spoke with Dan about the making of the Charlie Hebdo film, and what it’s like to continually work in this dark terrain:

Carol Nahra: You start out the film with an acknowledgement of the November attacks. How much did that tragedy affect the making of the film?

Dan Reed: The very last guy we happened to interview was the chief medical officer of the Paris fire service, who was at Charlie Hebdo and is one of the first people into the room. It was Friday the 13th of November, which is the date of the Paris attack, and we were chatting away at the end of the interview. I was saying “Something is going to happen again soon, I can feel it in my bones. It will either happen in Paris or London, there’s going to be another devastating attack soon. And there is no reason why it wouldn’t happen in a way, because nothing has changed to prevent it happening”. Literally, 200 metres from the studio where we shot our interview – which was our regular hangout in Paris where we shot most of our interviews – three or four hours later gunmen turned up and killed 19 people at a cafe on the corner. And the Bataclan was a short walk from Charlie Hebdo. My office in Paris was literally three metres away from the attack where Charlie Hebdo happened. I was working with that TV company (Premieres Lignes). So it all felt very very close… So we had to reference it back and say to people “look this is a film about what happened in November”.  And then we had to find a way in the preamble and the wrap up to make a distinction between the attacks.

CN: So much has been published in the media regarding Charlie Hebdo. What was your aim with this film?

DR: For one thing, to try and actually research the story properly, and figure out what exactly happened. We went into mind numbing detail about what actually happened, when and where. There is always drama in the two story of things…in the unfolding of events. There is often a lot of dead time, when people are waiting for police to arrive, and those are dramatic pauses…We did a lot of research to allow us to understand the drama of the story. We also got hold of a lot of images which had never been seen before – a lot of still images from the security cameras at Charlie Hebdo and the Jewish grocery. There are quite a few kind of scoops and untold bits in our story…So it’s kind of untangling the truth from the lies and the misperceptions and really establishing a proper timeline for the story, that took a lot of work. A lot of these people hadn’t spoken before, or hadn’t spoken at the time.

“There’s this strange process where you start from completely on the outside of events, and six, eight months later by the time you’ve corralled all these people together and got them to talk to you, you end up like a single point of contact for all these experiences.”

CN: Yes and they’re talking about very traumatic, harrowing and recent events. So what was that like?

DR: Again, there’s this strange process where you start from completely on the outside of events, and six, eight months later by the time you’ve corralled all these people together and got them to talk to you, you end up like a single point of contact for all these experiences…Every eyewitness is trapped in their often very narrow perspective. And often has a lot of misperceptions, a lot of questions, a lot of frustrating gaps that we’re able to fill in. So the satisfaction of being able to, if you like, piece together the narrative not only for filmmaking but also for sharing with the other victims – the survivors – that’s satisfying. I happen to speak French fluently, because I grew up speaking French. And that really helps. You’re immersed in this world of trauma and loss and people who can’t get these violent images out of their heads. It’s familiar territory I’m afraid.

CN: Can I ask you about Premieres Lignes. They’re your co-production partners, is that right? What was it like for them continuing to work in the same building?

DR: Really really hard. I don’t think I’m betraying confidence by saying there are a number of people within that company who would very much like to move, and of course it’s difficult and very expensive and may not even be a good idea. Very much to varying degrees some of them are definitely haunted by what happened and are reminded every day. It’s difficult not to be.

CN: It’s quite different from some of your other “Terror” films. Terror in the Mall had such abundant multi camera archive. Can you talk a little bit about the archive collection process for this?

DR: The key word is frustrating because I knew in particular that security camera footage existed from a number of locations where the attacks had happened. Because the footage was immediately impounded by the police, and because the prevailing attitude is “don’t let people see anything”, it was impossible to prise the moving pictures from the French authorities. And that was very frustrating because of course we would have used it responsibly.

“There is a huge world of difference between having something shocking in a twenty second clip on the web, and having it in a documentary where the people involved speak, and it’s done with care and compassion and sensibility.”

CN: So there’s a lot of footage that you couldn’t get?

DR: We just literally couldn’t get. There’s a really, really strong taboo in France against any images showing pain and suffering.  I found it kind of unhelpful in some ways…I think you can understand, but at the same time that really blocks a huge amount of journalism and seals off a lot of images. We live in a world where images are often the key to understanding situations. If they are used responsibly in the form of a longform narrative in particular then I think you can definitely justify the use of quite shocking images, if they’re in a context which creates understanding rather than used for just shock purposes. There is a huge world of difference between having something shocking in a twenty second clip on the web, and having it in a documentary where the people involved speak, and it’s done with care and compassion and sensibility. But no matter how you treat the material, the French are like not into that at all… Notwithstanding that I think we got a huge amount. It’s a more emotional story in a way than the others.

CN: Is doing film after film of darkness taking its toll on you?

DR: I don’t think I can do another one like this. I said this after Nairobi – I was being interviewed by the New York Times, saying “this broke my heart and I don’t think I can do another”. And here I am. But in fact I just turned down Terror in Paris 2 for the BBC, because I said “I can’t do this again. I can’t do this again in the same place.” The nature of the material, the darkness is enveloping, and you can kind of get lost in it. I think I can safely say I’m not going to do another blow by blow like these for a while.

Three Days of Terror: The Charlie Hebdo Attacks airs 6 January on BBC Two at 9pm.