Officials who are not supposed to talk about ongoing investigations, and might get fired if they do, but can't help themselves so they do it anyway under cover of "anonymity."

And honestly, probably everyone in a position to know, does know who the "anonymous" source is, but it's just enough plausible deniability that everyone gets away with it. They get to push their narrative but also pretend they are following the rules that are supposed to protect various parties in the process.

Meanwhile if I were on a grand jury and blabbing to the press every evening about an investigation, I could get in real trouble.