Chelsea Manning has been called to testify before a grand jury in the investigation of Julian Assange, officials said.

The summons is one of several indicators that prosecutors remain interested in WikiLeaks‘ publication of diplomatic cables and military war logs in 2010.

Prosecutors in Virginia have been pursuing a case based on conduct that predates WikiLeaks’ publication of hacked emails during the 2016 presidential campaign, and it is not clear investigators are interested in that activity.

Officials discussed the investigation of Mr. Assange, who founded WikiLeaks, on condition of anonymity because of the secrecy of the grand jury process.

Ms. Manning, whose subpoena was first reported by the New York Times, is a former Army private who served seven years in a military prison for passing secret State Department cables and military documents to WikiLeaks before receiving a commutation from Barack Obama.

Ms. Manning’s attorneys have filed a motion to quash the subpoena.

“I object strenuously to this subpoena, and to the grand jury process in general,” Ms. Manning said in a statement.

“We’ve seen this power abused countless times to target political speech. I have nothing to contribute to this case and I resent being forced to endanger myself by participating in this predatory practice.”

The subpoena was signed last month by Gordon Kromberg, a national security prosecutor on the Assange case. Mr. Kromberg last month persuaded a judge to leave sealed an indictment against Mr. Assange despite its inadvertent exposure in an unrelated court filing last year.

Under Mr. Obama, Justice Department officials had decided not to pursue charges against Mr. Assange and WikiLeaks after concluding that to do so could set a precedent that paved the way for prosecuting news organizations for publishing classified information. But the case got a fresh look under Donald Trump.

Steve Vladeck, a professor at the University of Texas at Austin School of Law, said the Justice Department probably indicted Mr. Assange last year to stay within the 10-year statute of limitations on unlawful possession or publication of national defense information and is now working to add charges.

“There’s nothing else that would make sense,” he said.

“The heart of the controversy is, there’s never been a successful prosecution” for publishing classified information, Mr. Vladeck said. “There has always been the specter of a First Amendment defense.”

Peter Zeidenberg, a national security defense attorney, said he cannot see grounds for Ms. Manning to refuse the subpoena.

“She’s already been prosecuted, she’s been convicted, she served a sentence,” he said. “She has no Fifth Amendment privilege over self-incrimination. If she doesn’t testify then she’ll be held in contempt.”

Ms. Manning appears to be the latest individual subpoenaed to testify before the grand jury investigating Mr. Assange in the Eastern District of Virginia.

Last July, computer expert David House, who befriended Ms. Manning in 2010 at a hackerspace in Boston he founded, testified for 90 minutes before the grand jury.

In an interview, Mr. House said he met the WikiLeaks founder in January 2011 while Mr. Assange was under house arrest at Ellingham Hall, a manor house 120 miles northeast of London.

Mr. Assange was fighting an extradition request by Sweden, where he faced an inquiry into allegations of sexual assault.

Mr. Assange asked Mr. House to help run political operations for WikiLeaks in the United States. “Specifically, he wanted me to help achieve favorable press for Chelsea Manning,” he said.

Mr. House, who testified in exchange for immunity, said the grand jury was interested in his relationship with Mr. Assange.

“They wanted full insight into WikiLeaks, what its goals were and why I was associated with it,” he said.

“They wanted explanations of why certain things occurred and how they occurred… It was all related to disclosures around the war logs.”

The grand jury seemed interested in whether Mr. Assange had solicited Ms. Manning to hack on WikiLeaks’ behalf, but did not press “very hard” on that, he said.

During her trial, Ms. Manning testified that she acted on her own to send documents to WikiLeaks and no one associated with WikiLeaks pressured her into giving more information.

He was not asked about WikiLeaks’ 2016 release of Democratic emails, which US intelligence agencies have assessed were hacked by Russians. Nor did he have personal knowledge of that, he said.

He was never told what charges prosecutors were contemplating.

Mr. House said his last contact with Mr. Assange was in 2013 and his last contact with WikiLeaks was in 2015.

Mr. House said he fears retribution for being associated with WikiLeaks and Ms. Manning in 2010 and does not believe the US prosecution is warranted.

“This is not an investigation borne out of a concern for national security,” he said. “It is an investigation borne out of retribution and revenge against Mr. Assange over the [2010] leak that he precipitated, and how this leak impacted the careers of politicians in Washington, DC”

Daniel Domscheit-Berg, a one-time WikiLeaks spokesman who has grown estranged from Mr Assange, said in an interview he was contacted by the German federal police in October 2017 and told that US authorities wished to talk to him “about the Manning-Julian connection”.

He also received a letter in March 2018 repeating the request from the then-US attorney in the Eastern District, Dana Boente.

He said the German police told him the FBI was interested in “what possible contact – possible coordination’’ occurred between Mr. Assange and Ms. Manning, he said. The Americans appeared to be interested in ”possible solicitation“ by Ms. Assange of Ms. Manning, he said.

Domscheit-Berg told the German police he was not interested in speaking to the FBI.