How in the world would Reuters know that the killings were done by "an Iranian-backed militia"?!?

The short answer is, they don't; Iran was simply a convenient country to blame for this escalation in violence.

Were I a betting woman, I would almost be willing to bet that those militia members doing the shooting and killing, were either American or Israeli military; this would be "the justification" for stopping the American withdrawal from the country, as its leadership is currently demanding.

Could it have been some of Muktada Al Sadr's people, former members of his Mahdi Army, which gave the US and Debathified Sunni militias such grief after the fall of Saddam Hussein?!? Possibly.

But again, why would Al Sadr's people give the US its excuse for staying in-country?!?

The guy may be many things, but stupid is absolutely not one of them; the fact that he is still breathing air, readily testifies to his mental acuity.