He is known only as “Profexer,” he wrote and sold code on the dark web then went dark himself last winter. In January, American intelligence agencies identified a piece of his code may have been used in the hacking of the Democratic National Committee.

Even though Profexer disappeared online, he appeared in real life, when he turned himself over to Ukrainian police and ultimately a witness for the F.B.I. His identity is being kept secret.

Right before turning himself in, he posted the following on a dark web server, “I don’t know what will happen… It won’t be pleasant. But I’m still alive.”

Why all the cloak and dagger stuff? While Profexer isn’t accused of assisting G.R.U. or Advanced Persistent Threat 28 (which Washington says is part of Russia’s intelligence services,) his code did.

Profexer (and his code) has become the missing piece of a puzzle that security experts have been trying to put together since December of last year. While all fingers have been pointing to Russia when it comes to hacking and interfering with the US election, everyone was stumped when the malware that was used pointed to the Ukraine and not to Russia. Serhiy Demediuk, chief of the Ukrainian Cyber Police, said “He told us he didn’t create it to be used in the way it was.”

It is still unclear what Profexer has told the F.B.I. or Ukrainian authorities.