I wouldnt rule out Jihadi's because they published a how to using pressure cookers manual on the internet a while back. They also don't have the capability to pull off anything big - if they could have they would have in the last 10 years.
While it is a tragedy in Boston, it's also taken over the news. On the same day 40 Iranians died in a Quake, about the same in Pakistan, and around 1000 lost their lives in the Syrian civil war. Life doesnt have the same reportable value in those parts though...
It's not just the British Library recording everything ever posted, the US Govt has done this for a long time. Google, Yahoo and Bing do it, but take a look at
Internet Archive: Digital Library of Free Books, Movies, Music & Wayback Machine and enter a website in the way back machine. That's a full copy of every website ever created since the 1990's. Fully functioning copies, clickable links the works, captured regularly. Great for prosecuting frauds, because taking down your fraudulent website won't help you as they grabbed it and copied it. Talk about storage capacity.....