Thanks, I heard the x-number of times thing, but I am not convinced as of yet it is valid.
I've found two threats and a warning to the Spanish government by the Spanish police, prior to the train attacks. The police warning may be referencing intelligence rather than a notice.
A new threat video was being recorded when the Spanish police were kicking the goblin's door down, according to a review of the tape. There was also another warning letter after the trains were attacked.
It appears that there are no rules whatsoever.