People have been using homemade explosives in New York City fairly regularly over the last few years, to include at the British Consulate in May 2005, at the Mexican Consulate in October 2007, at a Times Square Army recruiting in March 2008, and at an upper East Side Starbucks in May 2009. So far as I recall, there have been no arrests in any of those cases. To be sure, there were differences between those cases and yesterday's failed car bomb. The old cases involved small explosive charges that were detonated between the hours of 3:30 and 4:30 AM, presumably to avoid causing casualties, whereas the vehicle bomb was a large if highly eccentric assembly that fizzled and smoked at about 6:30 PM in a heavily trafficked location.
The most useful observation about the Times Square bomb that I've read so far was a piece re-tweeted by Marisa Urgo and written by Leah Farrall, a former senior Counter Terrorism Intelligence Analyst with the Australian Federal Police. According to a note on the failed [vehicle-borne improvised explosive device] in Times Square:
[W]hat strikes me about it all is the problem that repeatedly faces terrorists…actually getting something to go boom.
My first take is whoever did this didn’t have a whole lot of training, if any. And could have solely gone off manuals they’ve found on the net.
There are ample training materials out there from all manner of terrorist groups and crazies. And plenty of things that outline how to build a device just like this.
That said they knew enough to try to take identifying markers off the vehicle. However, this too can be found in a number of online guides.
Anyway, my point is that it is far more difficult to get something to go boom (for the average untrained person) than what people think. This is why, for example, training for construction of explosives and explosives devices in terrorist training camps has historically taken up to two years, as opposed to the usual basic training where people are trained how to *use* explosives instead of how to build devices. It is an ongoing problem for militant groups. This is why some of them (and here I’m thinking AQ) often sent the detonator or a key part of it back with those it was deploying to carry out attacks. Especially for the more sophisticated attacks. Or they gave intensive one on one or small group training. Not that this is the case here, but I point it out to reinforce the point that when groups or individuals don’t have training in construction of devices there is less likelihood their devices will detonate properly.
That's all that can be safely assumed right now. The bomber was an untrained person who was willing to cause mass casualties.