Islamist terrorist attacks around the world HAVE been done without claims of responsibility. In cases like that, it's either because they just believe that striking non-Islamist targets (including more moderate Islamic targets) are a goal in itself, or because their goal is simply to make the targets become more paranoid, insular, and security-bound.

The IRA always gave warnings so that they WOULDN'T kill people -- did the Unionists?

Basque separatists? SMERSH? Nazis who've remained hidden for the past sixty years?

Cobra Commander would have already claimed responsibility.