The hacker, known only by his online alias \u201cProfexer,\u201d kept a low profile. He wrote computer code alone in an apartment and quietly sold his handiwork on the anonymous portion of the internet known as the dark web. Last winter, he suddenly went dark entirely.<\/p>\n
Profexer\u2019s posts, already accessible only to a small band of fellow hackers and cybercriminals looking for software tips, blinked out in January \u2014 just days after American intelligence agencies publicly identified a program he had written as one tool used in Russian hacking in the United States. American intelligence agencies have determined Russian hackers were behind the electronic break-in of the Democratic National Committee.<\/p>\n
But while Profexer\u2019s online persona vanished, a flesh-and-blood person has emerged: a fearful man who the Ukrainian police said turned himself in early this year, and has now become a witness for the F.B.I.<\/p>\n
\u201cI don\u2019t know what will happen,\u201d he wrote in one of his last messages posted on a restricted-access website before going to the police. \u201cIt won\u2019t be pleasant. But I\u2019m still alive.\u201d<\/p>\n