This lawyer explains this whole thing, and touches on a the signal messages a little, I interpret what he said as he thinks someone was in the group from the government, but it's not totally clear: https://www.youtube.com/watch?v=NiCAFV_XBGc
So did the government come into possession of those messages by getting a warrant for the phones and discovering the messages post fact or is there some indication that the government was legally (or illegally) intercepting the messages over the wire?
I certainly know which one I think is likely.