I’ve thought on this some.
Most of us use SMS services that are provided by a wireless carrier like Verizon. I believe it to be a virtual certainty that all messages from these commercial SMS services are swept up by the NSA and available for searching for some period of time. I think you can also assume your emails on public servers like gmail and all your cell phone calls and voicemails are also swept up.
I have to question whether any federal agency uses a commercial SMS service from a national carrier for official business. Doing so would preclude virtually all communication between each other that has any classified, grand jury, confidential, ongoing investigation type information being transmitted.
It seems more likely to me these investigative and spook type agencies would use a private app that sent encrypted messages from a cell device over a commercial cellular network through a tightly secured firewall onto a server farm under the full control of the agency with no intermediate or local storage of data.
In short, the FBI could make those 6 months of text messages disappear if they wanted to. Individuals may subvert these controls and use personal or burner phones and there might be some data stored somewhere from these devices. Hard to imagine a Sr. Director of Counter Intelligence for the FBI is that stupid, although he was thinking with his small head during this time.
verizon retention of text messages-10 days