The aide, Alexander Vorobyov, told Moscow's Lefortovsky District Court on Friday that he worked as an assistant to President Vladimir Putin's envoy to the region, Nikolai Tsukanov. He was wearing a camouflaged ski mask while attending the court session.
The court ruled that Vorobyov be held in custody for two months as the probe continues. Earlier Friday, the Federal Security Service (FSB), the top KGB successor agency, said it arrested Vorobyov on treason charges that carry punishment of up to 20 years in prison, but didn't offer any details.
Vorobyov, 39, had worked as an aide to the presidential envoy for a year. Prior to that he had held various jobs in the provincial government of Russia's westernmost Kaliningrad region.