A report that the Justice Department has convened a rare special grand jury in Washington has raised questions about how the little-used investigative tool works and what, if anything, U.S. Attorney Jeanine Pirro plans to use the rare investigative move to advance.

Special grand juries can investigate alleged federal crimes, much like ordinary grand juries, but they carry a significant additional power. In limited circumstances, they can issue a public report about details from an investigation that are normally kept hidden from public view when prosecutors do not bring criminal charges.