That’s what Buttigieg indicated. There are secure means of communicating classified information, with a process for ensuring people with clearance are the only ones allowed to view this information. I imagine those rules are in place exactly for this reason, to prevent accidental leaks.

The rookies that Trump hired probably didn’t see the harm in doing it in this way, I mean come on, Signal is secure so what is there to worry about?

Who knows what else has been leaked to the wrong people accidentally.