WikiLeaks founder Julian Assange is fighting extradition from jail in the United Kingdom; he has been charged with violating the Espionage Act and conspiracy to hack into a government computer. (Matt Dunham/AP)

Jeremy Hammond, a hacker who admitted stealing files from an intelligence firm, has been brought to Virginia to testify before a grand jury that he believes is the panel investigating WikiLeaks and its founder, Julian Assange, Hammond’s representatives said in a statement.

The move is a new indication that prosecutors in the Eastern District of Virginia are expanding the WikiLeaks investigation even as Assange fights extradition from England.

“The government’s effort to try to compel Jeremy to testify is punitive and mean-spirited,” the Jeremy Hammond Support Committee said in a statement. “Jeremy firmly believes that grand juries are repressive tools of the government, used to investigate and intimidate activist communities and are abused by prosecutors to gain access to intelligence to which they are not entitled.”

A spokesman for the Eastern District of Virginia declined to comment Tuesday.

Hammond was moved over the weekend from federal prison in Memphis, the committee said. He was set to be released later this year but could serve more time because of the move, which imperils Hammond’s participation in a drug-treatment program.

Working with an anonymous-affiliated group called LulzSec, Hammond, a well-known Chicago hacker and anarchist activist, stole millions of internal emails from the intelligence firm Stratfor and released them to WikiLeaks, which published them in February 2012. The hack also exposed tens of thousands of credit card numbers of Stratfor clients, which include governments and major corporations that pay for analysis of international threats and trends.

He pleaded guilty to violating the Computer Fraud and Abuse Act in 2013 and was sentenced to 10 years in prison, with immunity from prosecution in other districts.

It’s unclear what authorities hope to learn from Hammond or how they would use the information. Assange is fighting extradition from jail in the United Kingdom; he has been charged with violating the Espionage Act and conspiracy to hack into a government computer. All the charges against him are related to WikiLeaks’ 2010 publication of war logs and diplomatic cables leaked by Army private Chelsea Manning.

A formal extradition request was issued in June, and grand juries cannot be used to bolster existing charges.

Nick Vamos, former head of extradition at the Crown Prosecution Service in England, said the treaty between the two countries still allows for the U.S. to add charges to the Assange case, but that will become more difficult and problematic for the American prosecutors as they get closer to the scheduled extradition hearing in February.

“Up until the point the district judge makes his or her decision, the U.S. can amend its request,” said Vamos. He cautioned that if the changes came after those preliminary hearings were held, “it would be considered by the defense and the judges extremely annoying, because you have to go back to the start. It’s very bad practice to do that.”

Prosecutors could question Hammond in connection with the December 2011 hack of Stratfor and any interactions he had directly with Assange.

“Charges against Assange for the Stratfor hack could even be charged as a separate case,” former U.S. attorney Barbara McQuade said. “Another possibility is charges against a co-defendant.”

An Icelandic hacker who worked with Assange at that time, Sigurdur Thordarson, has said he was interviewed by prosecutors after the May indictment. Thordarson in 2011 established a relationship with Hector Monsegur or “Sabu,” co-founder of LulzSec, who unbeknown to Thordarson was an FBI informant. An associate of Assange had reached out to Sabu online and sought to enlist LulzSec in hacking Icelandic government computers, according to former U.S. officials.

Manning is in jail in Alexandria, Va., for refusing to testify before a grand jury; prosecutors claim her testimony is “relevant and essential to an ongoing investigation into charges or targets” beyond Assange.

At his own sentencing in 2013, Hammond said he was inspired in large part by “the heroic actions of Chelsea Manning, who had exposed the atrocities committed by U.S. forces in Iraq and Afghanistan.”

He apologized for releasing “the personal information of innocent people who have had nothing to do with the operations of the institutions that I had targeted.” But he argued he was justified in targeting “information security firms because they work in secret to protect government and corporate interests at the expense of individual rights.”

He emphasized that it was another hacker who first broke into the credit card database. According to court documents, that hacker reached out to Sabu, who alerted Hammond to the vulnerability.

Along with Stratfor, Hammond admitted hacking the computer systems of several law enforcement organizations and defense companies. He also hacked into the websites of several foreign governments — under the direction, he said, of Monsegur.

Devlin Barrett contributed to this report.