Shiites Blamed for Bombing of Other Shiites - Iran?

Reuters: U.S. blames Shi'ite group ... for bombing and killing of 63 Shiites

Sure, it's easy to see they're trying to stir up sectarian violence. But if your sect is that important, why not blow up some Sunnis instead of your own Shiites? 3 possibilities:

1. It was accident.
2. Religion isn't as important in Iraq as it's made out to be. Or maybe the situation has evolved to that state.
3. The special Shiite cell was from Iran and didn't care so much for Iraqi Shiites.

Option 3 seems the most likely, for several reasons. See: Europe announces new Iranian sanctions during Bush visit