How could LP possibly know that CA put her hands around KC's neck. Someone who was there would have to have said something. Unless the neighbor was peeking in the windows???
On having seen GA's interview, it is possible that GA has spoken to the FBI about things that he has not felt comfortable revealing to LE. It's my impression that the information gathered by the FBI is not held to the same public scrutiny (sunshine law) as information gathered by LE. It's quite possible that more information has been divulged to them than to LE???
If it's true, I can't think how else that information could have come out...