Special counsel Robert Mueller, right, leaves after briefing senators on June 21 about his investigation of potential coordination between Russia and the Trump campaign. (Joshua Roberts/Reuters)

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.

In recent weeks and months, Mueller has been expanding the legal team working on the matter, and recently added Greg Andres, a longtime white-collar lawyer specializing in foreign bribery who previously worked in the Justice Department’s criminal division.

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.

Federal prosecutors had previously been using a grand jury in the Eastern District of Virginia, and even before Mueller was appointed, had increased their activity, issuing subpoenas and taking other investigative steps.

(Victoria Walker,Peter Stevenson/The Washington Post)

The Wall Street Journal on Thursday first reported the existence of the Washington grand jury.

A White House adviser said the president and his son-in-law, Jared Kushner, had not received subpoenas, nor had the White House. Members of the president’s legal team met with Mueller three weeks ago to express their desire to work with his investigators.

A spokesman for Mueller declined to comment for this article.

Ty Cobb, whom Trump appointed as White House special counsel, said of the grand jury: “This is news to me, but it’s welcome news to the extent it suggests that it may accelerate the resolution of Mr. Mueller’s work. The White House has every interest in bringing this to a prompt and fair conclusion. As we’ve said in the past, we’re committed to cooperating fully with Mr. Mueller.”

Mueller has largely removed the original prosecutors from the case, replacing them with a formidable collection of legal talent and expertise in prosecuting national security, fraud and public corruption cases, arguing matters before the Supreme Court and assessing complicated legal questions.

[As Mueller builds his Russia special-counsel team, every hire is under scrutiny]

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.

(Meg Kelly/The Washington Post)

It’s unclear why Mueller chose to use a panel in the District, although there are practical reasons to do so. The special counsel’s office is located in Southwest D.C. — much closer to the federal courthouse in the city than the one in Alexandria, Va. Mueller also had previously worked in the U.S. attorney’s office in D.C., giving him some familiarity with the courthouse and the judges.

Experts said that Washington would be the appropriate place to convene a grand jury to examine actions taken by Trump since he became president and took up residence at the White House. Many of the potential crimes Mueller’s team is investigating would have occurred in the District, such as allegations that Trump aides or advisers made false statements in disclosure records or lied to federal agents. The Post has previously reported that Mueller is investigating whether the president tried to obstruct justice leading up to his firing of Comey.

Others said the choice could reflect Mueller’s reputation for planning ahead and gaming out a possible trial. He could have better chances convicting aides to Trump in a city in which 90 percent of voters supported Democrat Hillary Clinton in 2016.

The special counsel team took over the investigation when Mueller was appointed in May, and prosecutors from the Eastern District of Virginia were largely taken off the case.

The only prosecutor known to have stayed was Brandon Van Grack, a national security division prosecutor whose name was on the subpoena connected to Flynn. Mueller’s team also absorbed an investigation of Manafort that was attempting to trace his sources of income and possible connections to the Russia case.

The grand jury in Virginia had issued a subpoena related to Flynn’s business, the Flynn Intel Group, which was paid more than $500,000 by a company owned by a Turkish American businessman close to top Turkish officials, said people familiar with the matter. A subpoena related to Manafort also was issued from Alexandria.

Rosalind S. Helderman, Spencer S. Hsu, Devlin Barrett and Tom Hamburger contributed to this report.