Washington CNN  — 

Special counsel Robert Mueller’s federal grand jury has been extended so it may continue to meet and vote on criminal indictments for up to six more months.

The grand jury’s initial 18-month term was set to expire over the weekend.

The extension is the surest sign yet that the Russia investigation isn’t finished. It means, broadly, that Mueller may continue pursuing alleged criminal activity related to the Russian government’s interference in the 2016 presidential election, and that more indictments may be coming.

Mueller is given authority by the Justice Department to prosecute individuals as he sees fit if they fall within his commissioned task – but in practice his team must ask the secret group of up to 23 citizens from DC to approve criminal indictments.

This grand jury, based in Washington, DC, and apparently dedicated to Mueller’s probe, began meeting in July 2017.

Since then the group has heard from dozens of witnesses in Mueller’s investigation and approved indictments of Trump campaign chairman Paul Manafort and his deputy Rick Gates, their Russian business associate Konstantin Kilimnik, 12 Russian military intelligence officers, and 13 Russians and three companies who allegedly manipulated social media to sway US voters. Manafort and Gates have since pleaded guilty to reduced sets of charges. One Russian company charged by Mueller, Concord Management and Catering, has pleaded not guilty, while the rest of the Russians have not appeared in US courts.

The group of grand jurors, which often meets on Fridays, has been visited by more than half of the prosecutors working for the special counsel’s office.

They last met, according to CNN reporting, on December 21 for about two hours.

In recent months, there have been several signs Mueller’s grand jury would press on.

Several associates of Trump adviser Roger Stone have received grand jury subpoenas. One associate is known to still be fighting a grand jury subpoena. Stone has not been charged with a crime, though has publicly stated he believes he will be indicted.

Separately, a foreign-owned company is fighting another grand jury subpoena apparently related to Mueller’s investigation, with a challenge currently before the Supreme Court. And the criminal case against Concord Management is related to an “unidentified matter occurring before the grand jury,” Justice Department prosecutors wrote to a judge on Thursday.

Under federal rules, the court is able to extend a grand jury’s term for up to six months if it is “in the public interest.”

It is not yet known how the federal government shutdown will affect the grand jury’s schedule. The federal court system has not yet run out of funding, but may by the end of next week.

Chief Judge Beryl Howell of the DC District Court, who oversees the Mueller grand jury, granted the extension. She does not sit in on its confidential sessions.

CNN’s Laura Robinson contributed to this report.