Special counsel Robert Mueller’s team revealed in a court filing Friday that it has evidence that Roger Stone, a major operative on President Donald Trump’s 2016 campaign, had communicated directly with WikiLeaks.

Though Stone’s January indictment, one of several to come out of Mueller’s investigation into possible collusion between the Trump campaign and Russian agents, accused him of being in touch with WikiLeaks in the lead-up to the election, Friday’s filing provides the strongest evidence yet against the longtime Trump confidant.

While investigating Russia’s 2016 hacking of Democratic Party documents, which were published on WikiLeaks, the special counsel’s office discovered “accounts that contained Stone’s communications with Guccifer 2.0 and with Organization 1,” which are aliases for WikiLeaks.

Prosecutors previously had evidence only that Stone had worked through intermediaries to attempt contact with WikiLeaks founder Julian Assange, find out what the hackers had stolen from the Democrats and determine how its release could help secure Trump’s victory.

Friday’s filing did not provide any additional details about what was included in those communications, which Stone has firmly denied. Last month, he appeared in federal court and pleaded not guilty to five counts of false statements, one count of obstruction of an official proceeding and one count of witness tampering.

Last February, The Atlantic reported that Stone exchanged direct messages with the WikiLeaks account on Twitter, in which Stone was asked to stop making “false claims of association” with the site. Friday’s filing, however, is the first time the special counsel’s office has confirmed evidence of direct communication between the two parties.