From the Washington Post:
Special counsel Robert S. Mueller III began using a grand jury in federal court in Washington several weeks ago as part of his investigation of possible coordination between the Kremlin and the Trump campaign, according to two people familiar with the inquiry.
The development is a sign that investigators continue to aggressively gather evidence in the case, and that Mueller is taking full control of a probe that predated him.
Mueller’s investigation now includes a look at whether President Trump obstructed justice by firing FBI Director James B. Comey, as well as deep dives into financial and other dealings of former national security adviser Michael T. Flynn and former Trump campaign chairman Paul Manafort.
In federal cases, a grand jury is not necessarily an indication that an indictment is imminent or even likely. Instead, it is a powerful investigative tool that prosecutors use to compel witnesses to testify or force people or companies to turn over documents.
The assembling of a grand jury does not necessarily mean that an indictment is certain. But, it allows the Special Counsel to subpoena individuals to testify under oath and to force individuals or companies to turn over documents.
The New York Times reports that subpoenas have been issued in regard to the June 9, 2016, meeting at Trump Tower between Donald Trump Jr., Jared Kushner and a “Russian government attorney,” as well as business documents from former National Security Adviser Michael Flynn.