On thread #2, someone estimated Christensen to have been online until right before his arrest, based on the information ("Last Online") from his Steam profile.
Could he have "confessed" to someone he had been voice-chatting with over Steam? Perhaps he thought no one would be able to figure out who they were talking to, so he let it slip?
If he was using a relatively long-range wireless headset in particular, it probably wouldn't have been that difficult for the FBI to eavesdrop on the conversation, without having to bug the whole apartment or having someone wear a wire.
I considered this as well... I'm leaning toward an in person informant though. I have heard, but am not sure, that it takes quite a lengthy legal process to surveil without permission. It would be much quicker if someone keen to consent was involved. Jmo.