Anders Breivik, who confessed to killing 77 people in Norway in July, has reappeared in court. Every time I see his face on TV a chill runs down my spine.
This may be too emotional a response for a journalist, since strong feelings can arguably compromise impartiality.
Having interviewed some of the young survivors of Utoeya, it is perhaps unsurprising that I feel some empathy towards them and anger towards him. But my reaction isn't just caused by my empathy with his victims.
I was on shift at 6am in the BBC newsroom on Saturday 23 July, the day after he committed the massacre. It wasn't until late on the Friday that the horror of his killings in Utoeya emerged, since they were in part eclipsed by the bomb attack in Oslo. With the full scale of the massacre revealed, I joined the crowds on the internet digging for information about the killer.
His name was uncommon - the Norwegian White Pages online just had one Anders Behring Breivik in the whole of Norway. His Twitter account - that has now been hacked with wilful humour by Anonymous - was easily found and his only tweet was a John Stuart Mill quote: "One person with a belief is equal to the force of 100,000 who have only interests." Along with audience reaction and eyewitness accounts, I fed this into the BBC's live coverage of events to help profile the killer.
Some commentators were arguing that Breivik's social media presence was suspiciously fresh, due to his Facebook and Twitter profiles being new. But if you searched for his name within Norwegian web domains (in Google: "Anders Behring Breivik" site:no) you could see entries going back to October 2010 on document.no, a Norwegian right-leaning online political community. I spent some time verifying that this was the same Breivik, deciphering his thoughts on Google Translate and publishing indications of them on our BBC internal system.
We also got information about Breivik from our audience. A couple of online gamers contacted us via the News website to share their experiences of playing World of Warcraft with him. Breivik's username on the site was 'Conservatism' and there were (and still are, though some have been deleted) threads about him on the site's forums. He apparently showed "intelligence" and "an ability to assemble data and information... and deliver excellent tactics".
Breivik's YouTube manifesto published under an anglicised username, BerwickAndrew, was 'found' and widely circulated later in the evening, despite YouTube's attempts to take it down. On Sunday, I downloaded his 1,500-page written manifesto in the same name and scanned it for his opinions not just on Europe and Islam but diets and steroids. I found the document on a file-sharing site and, in the short term, there was little way to prove without doubt it hadn't been tampered with. Yet, notwithstanding the new elements, the content seemed in line with everything else Breivik had posted.
Throughout the weekend, the question lingered as to whether Breivik had acted alone. The Norwegian paper VG reported eyewitnesses suggesting there had been more than one killer, and some experts questioned how one man could cause such devastation.
But, having immersed myself in the virtual world of Breivik, I had a strong sense that he was a man who had acted on his own. In fact, I felt sure he had. I wouldn't of course broadcast a hunch, but it stopped me from (for example) re-interviewing eyewitnesses to ask more questions.
Getting under Breivik's skin may have also helped me to follow the breadcrumbs he'd laid out online safely and faster than if I hadn't.
So the chill in the spine may be worth it... although getting inside the mind of a killer is a little disturbing.
Silvia Costeloe is a user-generated content and social media journalist at BBC News.