WaPo:
According to court records, during a security-clearance renewal last year, [he] failed a polygraph test and subsequently confessed that he had been in contact with Russian intelligence for 15 years.
The contact started in 1996 when he was studying abroad in Chelyabinsk and met his future wife, the daughter of a colonel in the Russian air force. Over the next decade, as he rose in the U.S. Army ranks, [he] shared information on his Special Forces unit’s activities and personnel, the names of U.S. counterintelligence operatives and the name of a fellow Special Forces member he thought might be receptive to recruitment by Russia, according to court papers.
CBS:
[He] had a 15-year relationship with Russian intelligence, dating back to 1996 when he was an ROTC student at the University of Minnesota and on a visit to Russia for an independent study program gave a handler there the names of four Catholic nuns he had visited, according to the charges against him. Shortly thereafter, Russian agents assigned him a code name, "Ikar Lesnikov."