I am not saying it was Israel (although, for the first time in a long time, I agree with Mongo on something, which is scary). It could be something internal, though that is less likely. The reason is that plenty of people in Iran want their current government (or governments, there are three) gone. That has been true for the last thirty-plus years now (so two generations that hate the system).
So, if the attack had been different, I might very suspect something like the radical Revolutionary Guards (think SS Officers) vs. The Regular Military (or vice versus), anyone Versus the Mullahs or the New, Improved, Not Your Grandfather's Revolution Freedom Underground (I just made that up).
But someone will likely claim responsibility soon if it is internal or a Sunni State/Non-State actor. The problem with "internal" is that Sulimani seems to have been a popular figure in Iran and seen as a war hero by many, even those who might not approve of their government much. Killing a hundred or more civilians waiting in lines of thousands (one source said up to 10,000 plus) doesn't seem like a great way to kick off an internal revolution.
However, things are so hot and crazy right now that anyone with a match might be willing to try to kick things off. Iran, aka Persia, is much more complicated than most Americans (or Europeans) think. This isn't a place that was goat herders and camel drivers until seventy years ago. These are the original "Aryans," an Indo-European culture. One is temporarily taken over by Islamic fanatics, but their basic worldview is anything but Arab.