Why Mueller’s use of a grand jury confirms what we already knew
The grand jury has the subpoena power that prosecutors need to compel reluctant witnesses to testify under oath. Grand jury subpoenas are also how prosecutors gather documents such as bank records, emails and corporate papers from entities or people who might not produce them voluntarily.
.. But someone who receives a subpoena to testify or produce documents is not bound by those secrecy rules. They are free to disclose — to the media or to anyone else — that they received a grand jury subpoena or testified in the grand jury. It may be that someone who just received a subpoena contacted a reporter and that has resulted in the “breaking news” stories.