While that's true, I think two factors strongly indicate that this was targeted at the ATT building. The first observation is, of course, that the ATT building was the closest to the explosion. A direct witness to the RV broadcasting the evacuation message, who lives across the street, describes this fact: https://www.youtube.com/watch?v=iag6cTWpgq8. The second observation is the evacuation message itself. How often do we hear about bombings where the bombers broadcast an evacuation message on a loudspeaker for 15 minutes prior to the explosion? Telling people to evacuate doesn't sound like the kind of thing you would do if your primary intent was to inflict loss of life.