WASHINGTON - A jailed Russian who says he hacked into the Democratic National Committee computers on the Kremlin’s orders to steal emails released during the 2016 U.S. presidential election campaign now claims he left behind a data signature to prove his assertion. In an interview with Russia’s RAIN television channel made public Wednesday, Konstantin Kozlovsky provided further details about what he said was a hacking operation led by the Russian intelligence agency known by its initials FSB. Among them, Kozlovsky said he worked with the FSB to develop computer viruses that were first tested on large, unsuspecting Russian companies, such...