Washington (CNN) The grand jury's criminal investigation started by special counsel investigators is "continuing robustly," a federal prosecutor said Wednesday, even though Robert Mueller filed his report on Russian interference in the 2016 election with the Justice Department last week.

In a federal court hearing about whether the name of a foreign-owned company and the country that's been fighting a subpoena from Mueller since last summer should be revealed, Chief Judge Beryl Howell asked prosecutor David Goodhand point-blank if the grand jury investigation is over.

"No, it's continuing. I can say it's continuing robustly," said Goodhand, an assistant prosecutor in the DC US Attorney's office.

It remains unclear what part of Mueller's investigation is ongoing and if more criminal charges could still come, but the hearing on Wednesday made clear that stones Mueller turned over in the last 22 months will continue to be part of federal court proceedings and possibly lead to new cases.

There were no prosecutors from Mueller's office in court. The special counsel's office has handed over the subpoena case to the DC US Attorney's office.

The company has hired US-based lawyers from a private law firm to fight in court on its behalf, and even the foreign country that owns it isn't known. The company's matter amounts to one of the most mysterious but closely followed aspects of Mueller's probe, as its subpoena challenge has progressed through the court system since last fall.

The continuing work of prosecutors, even after Mueller declared his investigation had concluded, further raises questions about what the Mueller investigation touched on in addition to the central questions of the Russian hack of the Democratic Party; a Russian conspiracy to flood American social media with propaganda; Trump campaign staffers' communications with Russians in 2016; and whether the President obstructed justice by firing the FBI director and with public statements he made about the Russia investigation.

Mueller did not establish collusion between the Russian government and the Trump campaign, and he declined to decide whether evidence could lead to an obstruction charge against Trump, Attorney General William Barr said this weekend. Barr and Deputy Attorney General chose not to prosecute the President for obstruction because the evidence wasn't sufficient, Barr said in a letter to Congress summarizing Mueller's findings.

The special counsel has previously said the company he had subpoenaed is not a target of the investigation but has information needed by prosecutors.

Howell did not make a decision Wednesday, but acknowledged that it would be harder under the law to reveal the company's identity if there continues to be an active investigation.

The chief judge had spoken with both the mystery company's attorneys and with the Justice Department prosecutors in a closed hearing apparently lasting more than an hour prior to the public hearing Wednesday.

As soon as members of the media and other court-watchers filled the courtroom, Howell asked the company to clarify if it wanted its identity to stay secret.

"My client" doesn't want its identity revealed, said Brian Boone, a lawyer from the law firm Alston & Bird. Howell asked for a reason, and Boone said he'd didn't want to say in open court. Then he and three other lawyers from the law firm packed their bags and beelined out of the court room -- before Howell could even acknowledge they were excused.

The judge then heard from Ted Boutrous, representing the Reporters Committee for Freedom of the Press, on why more redacted documents in the case should be made available, and whether to name the company. Prosecutors from Mueller's team who've fought over the subpoena in multiple courts didn't attend on Wednesday; instead it was three federal prosecutors from the DC US Attorney's Office asserting that what started as the Mueller investigation continues.

The foreign-government-owned company has been held in contempt and is accruing daily fines of $50,000 since mid-January for not turning over the documents requested.

Howell has made a limited amount of documents in the mysterious case available, including her opinions in the District Court that the company couldn't resist the subpoena even if it was owned by a foreign government and that it should face daily mounting fines for noncompliance.

"There should be no secret law," Howell said Wednesday.

The company, which says it is wholly owned by a foreign government, has lost its attempts to throw out Howell's orders on appeal, including with a recent request that the Supreme Court turned away.

CLARIFICATION: This story was updated to more precisely reflect Attorney General Bill Barr's letter to Congress. Specifically, Barr quoted from special counsel Robert Mueller's report to say the "investigation did not establish that members of the Trump Campaign conspired or coordinated with the Russian government in its election interference activities."