He is Dmitry Firtash, a former fireman and soldier. In little more than a decade, the Ukrainian went from obscurity to wealth and renown, largely by buying gas from Russia and selling it in his home country. His success was built on remarkable sweetheart deals brokered by associates of Russian leader Vladimir Putin, at immense cost to Russian taxpayers, a Reuters investigation shows. Russian government records reviewed for this article reveal for the first time the terms of recent deals between Firtash and Russia's Gazprom, a giant gas company majority owned by the state. According to Russian customs documents detailing...