Who Shot Down the Airliner?

The Americans say the Iranians shot down the Ukrainian jet today, bound for Kiev, which exploded shortly after takeoff.
Excuse my skepticism, but after WMDs, the Kuwaiti incubator story, the Gulf of Tonkin resolution, and so on, I’m inclined to believe the Americans are lying. At any rate, I’m going to need a lot more than “U.S. officials are confident…” or “17 U.S. intelligence agencies agree…” Well, of course they do. Their job is to agree and I’m pretty sure at least 16 of them are totally redundant.
The Iranians have denied it completely. Ali Abedzadeh, head of the Iranian Civilian Aviation Association said it was ‘physically impossible’ and a ‘ridiculous charge.’ Well, that’s not entirely so, either. If a missile was fired and it struck the plane, that is entirely physically possible. It is the reason anti-aircraft missiles exist.
Still, I can’t see why Iran would have any motive. It looked like WWIII had been averted, cooler heads were beginning to prevail, so why?
Some people are saying it may have been an accident. Pretty tragic, but possible. If the missiles were loaded and ready, because of the state of emergency caused by Soleimani’s assassination, then all it would take is somebody setting their coffee down on the wrong button and uh-oh, international incident.
So, which ones should we believe? We should probably wait for the aircraft investigations team to finish their investigation, and stop listening to politicians.
FWIW, here’s my solution: nothing was done about it when the U.S. shot down an Iranian civilian plane a few years back. Let’s treat this incident exactly the same.