Washington (CNN) The Mueller investigation witness longest known to have refused testifying finally spoke to a secret federal grand jury Friday about conservative political operative Roger Stone , the 2016 Republican National Convention and his relationship with Stone since then and will give more documents to investigators in the coming week, his attorney said.

The session indicates that a federal grand jury previously used in the Mueller investigation is still interested in Stone, and new charges or cases could be on the horizon. Separately on Friday, prosecutors made their boldest statement yet that they looked closely at whether Stone had violated the law against hacking while he was in contact with Russians online and WikiLeaks in 2016.

Miller also testified Friday about what he knew of Stone's contact with Julian Assange and WikiLeaks. The documents prosecutors have requested pertain to Stone's schedules during the 2016 political convention.

"It's hard to say where they're going on this," his attorney Paul Kamenar said outside the courthouse about the ongoing investigation, which has an unknown scope but was previously handled by special counsel Robert Mueller

"There could very well be a continuing investigation" of Russian interference in the 2016 election, but Miller did not have much information to help prosecutors, Kamenar said. Miller did not know of contact between Assange and Stone, Kamenar said.

Stone and Assange have already been charged with crimes, and Stone has pleaded not guilty to obstruction, witness intimidation and lying charges. The grand jury at this point is not able to use Miller's testimony to build those open cases, and instead under Department of Justice policy the grand jury must work toward new charges.

Miller said he worked for Stone -- "Uncle Roger" he called him after his grand jury session Friday -- for 13 years, as a driver and helping to manage Stone's emails and website. He has not worked with Stone since the 2016 election, his attorney said.

He does not need to testify again to the grand jury, his attorney said he was told.

Miller held off testifying for a year as he challenged the constitutionality of Mueller's investigation, and the courts denied him.

Searches of Stone related to 2016 hack

Hours after Miller testified, prosecutors told a federal judge via a court filing they had used 18 search warrants to look into possible criminal activity around Stone's communications with Assange, WikiLeaks and Russians who used the online pseudonym Guccifer 2.0 as they distributed stolen Democratic documents intended to help Donald Trump's campaign.

Prosecutors underline several times in this new filing that Stone had indeed been in contact with WikiLeaks and Assange. They had not made that assertion in his indictment, and he has not been charged with any crimes related to contacting WikiLeaks or Russians, or with hacking.

Previously, prosecutors had emphasized only how Stone allegedly used intermediaries to try to reach Assange as a way to help Trump's campaign -- but hadn't fully explained whether anyone had made contact with the offshore group. Stone allegedly tried to cover up to Congress his discussions with the Trump campaign and his helpers as they sought to reach Assange.

"Stone communicated with the very organization that was disseminating materials from the DNC computers in the months after the hack," prosecutors wrote in the filing Friday. "Based on those communications, there was probable cause to believe that evidence related to the DNC hack would be found," they added, describing how Stone communicated with Guccifer 2.0, WikiLeaks and Assange.

In 14 of their search warrant affidavits, prosecutors say they outlined how they believed searches of Stone would turn up evidence that he "intentionally" accessed computers illegally, according to the new filing.

There is one known exchange of messages between WikiLeaks and Stone. In February 2018, the Atlantic reported Stone had exchanged direct messages via Twitter with the WikiLeaks account in which he was asked to stop associating himself with the site. Both denied they were in contact about the release of Hillary Clinton emails.