Before diving into the possibilities that I see, a discussion of the device is in order. The construction of the device was, in a word, childish, as if it was assembled by a 12 year old with pyromania. I am basing my opinion on the description of the device by NYPD Commissioner Kelly. He described the device as consisting of two 5-gallon canisters of gasoline (evidently not pressurized, and therefore merely flammable) with an 'explosive' between them (M-88 firecrackers evidently serving as primers throughout the VBIED). These were sitting in the back seat. Behind the seat in the cargo area were 3x ~15 pound standard propane tanks of the type that you use on your barbeque grill. One of these had M-88's strapped to it (rather ineffective primer I think). Behind that a metal gun case (dimensionally probably a rifle case of the type you check weapons under the plane in) filled with several 'bags' of an unidentified chemical substance (probably fertilizer per Kelly, and no hints that it was soaked in fuel oil). According to Kelly the system was functioning, referring to the wiring and potential to actually detonate. Taliban claims to the incident are likely unfounded (http://www.ny1.com/1-all-boroughs-news-content/top_stories/117974/nypd--taliban-claims-unfounded-in-times-square-bombing-attempt).
The device evidently would have functioned by first igniting the gasoline, then the ensuing blaze detonating the propane (assuming the M-88 was ineffective), and then detonating the presumed fertilizer in the gun case. This sequence of events could conceivably take only minutes; however, that leaves plenty of time for a warning to be raised (as it was) and people to get safely away. This in and of itself makes the device orders of magnitude less efficient than an instantaneous device. Also, the vast majority of the device was constructed using materials that deliver an impressive fireball but very little oomph (the effects are NOT directly related). These points beg the question: did the perp want the device to be discovered before it hurt anybody?
The ultimate point of terrorism is to scare people into pressuring their government for political change. In this case the message seems to be that it is still possible, even in our current world focused on homeland security, for significant civilian targets (i.e. Times Square) to be hit. The unlikely possiblity exists that this bomb scare was perpetrated by someone simply trying to remind us of that for our own good. This perverse altruism is reinforced by the apparent 'slow' and exposed design of the device (lending itself to discovery and deactivation), and would almost certainly be a product of some delusional psychopathology. Regardless of motive, the individual amateur theory is reinforced by the poor design of the VBIED itself. If they were to operate in a group/cell, they would be far more likely to produce a better bomb (someone in the group could have at least Googled the proper way to build a bomb), and the apparent lack of group involvement lends support to psychopathology. It is important to note that disorders such as Schizophrenia do not necessarily preclude higher brain planning functions and so on. Sub-clinical diagnosis is also possible, or an individual disturbed enough to want to lash out at society (a la Timothy McVeigh). In the latter case, the intended message would more likely be to protest against the excesses of commercialism.
If this was in fact perpetrated by a group, the likely motive would be to observe and analyze the police response for use in future 'real' operations, and/or to provide a distraction and resource sink for the NYPD while something happens elsewhere.
While I am just speculating and rambling, the important thing to take from this is to keep your eyes peeled for things that are out of place and don't be bashful about alerting the authorities to them.
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