Nick Schifrin:

So there are questions about the system that Iran was using and questions about user error.

So, first the system, Judy. The U.S. intelligence assessment is that this was an old Russian short-range missile defense system called Tor or SA-15. The hardware on that system is considered relatively reliable, but the computer software is not as sophisticated.

And it's that software, that it's not about targeting, but it's about knowing which plane is in the sky. And so the experts I spoke to — and they are just experts. They're outside of the government. Experts I spoke to said maybe the software couldn't determine which kind of plane it was.

And that's where you get user error. The time the operator of the system had to decide what to do was seconds. The operator is sitting there just hours after that major Iranian attack on U.S. bases in Iraq, fearing that there could be U.S. jets, military jets, in the sky.

And this is the fundamental nature of conflict, Judy, that miscalculations can happen. And this happened, by the way, to the U.S. 1988, U.S.-Iran tanker war, the USS Vincennes — it's a warship — was skirmishing with Iranian boats in the Persian Gulf, and had actually accidentally entered Iranian waters, when it discovered or thought it had discovered an Iranian military jet coming toward the ship.

That was actually a civilian jet, but it didn't know that. It didn't quite know what it was. And so it fired; 298 people died in that mistake that the U.S. has made in the past.