Howell declined to comment on how much longer the grand jury could sit. Under federal rules of criminal procedure, a grand jury may serve more than 18 months only if a judge finds an extension is in the public interest, and then generally for no more than six additional months.
The extension comes amid suggestions the Mueller probe might be drawing to an end.
Since Mueller’s appointment in March 2017, the special counsel’s investigation has led to charges against 33 people, including 26 Russians.
Mueller’s probe became a lightning rod for criticism by the White House and its Republican supporters.
The future of Mueller’s investigation was also thrown into question after November’s midterm congressional elections, after Trump ousted Attorney General Jeff Sessions and elevated Matthew G. Whitaker to oversee the probe.
In one of their first moves upon taking control of the U.S. House of Representatives this week, Democratic lawmakers reintroduced legislation Thursday to protect Mueller if Trump attempts to fire him, by permitting Mueller to appeal such an order to a panel of judges.