It certainly seems to be a possibility. Reading over Bill Roggio's blog I find it hard to see a flaw i his reasoning. But this raises an important question.
Given the USA has been blaming Iran for damn near everything in Iraq recently, why is it waiting on a Pentagon investigation before accusing anyone? Could it be the USA was bluffing and so does not in fact have detailed (or any) knowledge of Iranian operations in Iraq? And more importantly, are Iran calling their bluff as well as taking revenge for the recent consulate raid?
The MO suggests this, given the attack was carried out by troops who could pass for Americans in look and language ability, yet the bodies were found dumped by a Sunni neighbourhood. That makes no sense, as there are few Sunni groups who could operate so precisely in Karbala.
Either way, the Iranians are likely to be cleaning up possible informants in southern Iraq for this op over the next few weeks, and the USA has authorized the death of captured Iranian agents. So expect a bloody few weeks in Shiite Iraq.