Hacker “Guccifer 2.0,” who stole Democratic National Committee emails that turned the 2016 presidential election upside down, has been identified as a Russian military intelligence officer thanks to an incriminating clue he accidentally left online, according to an explosive report Thursday.

The real identity of the DNC hacker was uncovered by a group of FBI agents, who have now been brought on board special counsel Robert Mueller’s team as he probes Russian interference in the 2016 election, the Daily Beast reported.

Roger Stone, a former campaign adviser to President Trump, had previously insisted the hacker wasn’t Russian – just a lone-wolf Romanian who did the DNC hack as a lark.

Stone has boasted that he’s been in contact with Guccifer 2.0 during the 2016 campaign and repeated the hacker’s story that he’s Romanian with no ties to Russia. Guccifer 2.0 became his own media personality via a WordPress blog and Twitter, always sticking to the Romanian line.

Cyber sleuths looking at metadata in the hacker’s social media always hit a dead end in the form of Elite VPN, a virtual private networking service that obscured Guccifer 2.0’s real location, the Daily Beast reported.

But Guccifer 2.0 slipped up a single time and failed to activate the VPN, exposing his actual Internet Protocol address in Moscow, a source familiar with the government’s hunt told The Daily Beast.

The cyber morsel was enough to ID Guccifer as a GRU officer working out of the agency’s headquarters on Grizodubovoy Street in the Russian capital, the Daily Beast said.

“Almost immediately various cyber security companies and individuals were skeptical of Guccifer 2.0 and the backstory that he had generated for himself,” said Kyle Ehmke, an intelligence researcher at the cyber security firm ThreatConnect.

“We started seeing these inconsistencies that led back to the idea that he was created hastily… by the individual or individuals that affected the DNC compromise.”