Like I said in another thread, I'm not sure that it was a terrorist attack. Why? Because the bomb went off when there were very few people in the streets, and a loudspeaker on the RV announced that a bomb was about to go off and that people should evacuate.
Then, there is the fact that it went off at a communications hub for the city, knocking out communications of all sorts in a lot of areas, and the airport was shut down for several hours because of that.
Me? If I'm gonna go conspiracy theory, I'd be willing to say that it was a smokescreen and a means to knock out communications while another crime was being committed. Police should be looking into other crimes that happened around that time, and if they required communications to be knocked out, that would be your most likely suspects.