The article points out that it's unknown how the feds accessed the messages while noting the possibility someone who was party to the group may have provided direct access. This seems the most likely explaination given the circumstances and absent any credible evidence of actual compromise to the protocol. An accomplice looking for a lighter sentence (...or one who is simply incompetent) will almost always be the weaker link in secure communications.

Damn. At least it was federal investigation and not some copyright bullshit, but still...