If its a criminal act...
....do the authorities/countries involved have to report what they know. How much information are they obliged to release?
....if it was a criminal act by a radical group - they would have let us know within hours that they were responsible to bask in their infamy.
....if it was the sad act of an individual who had the extensive knowledge and cunning to perpetrate this - surely that person would want the world to know that they were that clever/sick! They would have left some note, tweet, Facebook clue. I feel they could not have the self-control to leave no clue at all - unless the authorities have that clue and want to keep it to themselves.
....or it was an insurance job and had to be scrupulously covered up.