A jailed Russian hacker claims he can prove he broke into the computers of the Democratic National Committee on behalf of Russian intelligence — because he left a secret calling card inside the system.

In an interview Wednesday with Russia’s independent TV Rain, Konstantin Kozlovsky said he inserted a .dat file imprinted with his passport number and the number of the visa he got for a trip to the Caribbean island of St. Martin.

“If the Americans made snapshots, you can check it out,” he said in a written exchange with the outlet from his Russian jail cell. “Or the file should be on backups for September 2015. That’s the main proof I can provide from inside this dungeon.”

After the intrusions allegedly took place, from summer 2015 to early 2016, Kozlovsky was imprisoned on charges of being part of a hacking plot to steal millions from Russian banks. This month, reports began to circulate in Russian media that Kozlovsky had confessed in Russian court in August to being part of an operation run by the country’s intelligence service, the FSB, against the DNC.

Kozlovsky has posted what appears to be a court transcript of that confession to Facebook, along with a corresponding audio recording of what sounds like a court hearing on SoundCloud.

Now, he’s claiming he left a trail — but it’s yet to be independently confirmed.

In the wake of the alleged break-ins, the DNC hired cybersecurity firm CrowdStrike to investigate. The firm eventually concluded that two Russia-based outfits — known as Fancy Bear and Cozy Bear — were responsible for the hack.

CrowdStrike didn’t immediately respond to a request for comment from VICE News.

Russian President Vladimir Putin has repeatedly denied his intelligence services waged a cyber campaign to influence the U.S. 2016 election, despite a consensus among American intelligence agencies that they did.

Kozlovsky claimed to have infiltrated the DNC server under the direction of FSB officer named Dmitry Dokuchayev. Dokuchayev himself is somewhat of a controversial character.

In March, a grand jury in the Northern District of California indicted Dokuchayev for allegedly hacking 500 million Yahoo accounts and then using that information to access the accounts of “Russian journalists, U.S. and Russian government officials, and private-sector employees” at financial companies.

That announcement was made by Attorney General Jeff Sessions and then-FBI Director James Comey, and an FBI “Wanted” poster was released for Dokuchayev the same day.

But according to Russian press reports, Dokuchayev had already been arrested in Russia in December on charges of treason — on suspicion of passing information to U.S. intelligence agencies.