– The hacker, known only by his online alias “Profexer,” 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.

Profexer’s posts, already accessible only to a small band of fellow hackers and cybercriminals looking for software tips, blinked out in January — just days after U.S. intelligence agencies publicly identified a program he had written as one tool used in the hacking of the Democratic National Committee.

But while Profexer’s online persona vanished, a flesh-and-blood person has emerged: a fearful man who Ukrainian police said turned himself in early this year and has now become a witness for the FBI. “I don’t know what will happen,” he wrote in one of his last messages posted on a restricted-access website before going to the police. “It won’t be pleasant. But I’m still alive.”

It is the first known instance of a living witness emerging from the arid mass of technical detail that has so far shaped the investigation into the DNC hack and the heated debate it has stirred. Ukrainian police declined to divulge the man’s name or other details, only saying that he is living in Ukraine and has not been arrested.

There is no evidence that Profexer worked, at least knowingly, for Russia’s intelligence services, but his malware apparently did.

That a hacking operation that Washington is convinced was orchestrated by Moscow would obtain malware from a source in Ukraine — perhaps the Kremlin’s most bitter enemy — sheds considerable light on the Russian security services’ modus operandi in what Western intelligence agencies say is their clandestine cyberwar against the United States and Europe.

It does not suggest a compact team of government employees who write all their own code and carry out attacks during office hours in Moscow or St. Petersburg, but rather a far looser enterprise that draws on talent and hacking tools wherever they can be found.

Also emerging from Ukraine is a sharper picture of what the United States believes is a Russian government hacking group known as Advanced Persistent Threat 28, or Fancy Bear. It is this group, which U.S. intelligence agencies believe is operated by Russian military intelligence, that has been blamed, along with a second Russian outfit known as Cozy Bear, for the DNC intrusion.

Rather than training, arming and deploying hackers to carry out a specific mission like just another military unit, Fancy Bear and its twin Cozy Bear have operated more as centers for organization and financing; much of the hard work like coding is outsourced to private and often crime-tainted vendors.

Russia’s testing ground

In more than a decade of tracking suspected Russian-directed cyberattacks against a host of targets in the West and in former Soviet territories — NATO, electrical grids, research groups, journalists critical of Russia and political parties, to name a few — security services around the world have identified only a handful of people who are directly involved in either carrying out such attacks or providing the cyberweapons that were used.

This absence of reliable witnesses has left ample room for President Donald Trump and others to raise doubts about whether Russia really was involved in the DNC hack.

“There is not now and never has been a single piece of technical evidence produced that connects the malware used in the DNC attack to the GRU, FSB or any agency of the Russian government,” said Jeffrey Carr, the author of a book on cyberwarfare. The GRU is Russia’s military intelligence agency, and the FSB its federal security service.

U.S. intelligence agencies, however, have been unequivocal in pointing a finger at Russia.

Seeking a path out of this fog, cybersecurity researchers and Western law enforcement officers have turned to Ukraine, a country that Russia has used for years as a laboratory for a range of politicized operations that later cropped up elsewhere, including electoral hacking in the United States.

In several instances, certain types of computer intrusions, like the use of malware to knock out crucial infrastructure or to pilfer e-mail messages later released to tilt public opinion, occurred in Ukraine first. Only later were the same techniques used in Western Europe and the United States.

So, not surprisingly, those studying cyberwar in Ukraine are now turning up clues in the investigation of the DNC hack, including the discovery of a rare witness.

Security experts were initially left scratching their heads when the Department of Homeland Security on Dec. 29 released technical evidence of Russian hacking that seemed to point not to Russia, but rather to Ukraine.

In this initial report, the department released only one sample of malware said to be an indicator of Russian state-sponsored hacking, though outside experts said a variety of malicious programs were used in Russian electoral hacking.

The sample pointed to a malware program called the PAS web shell, a hacking tool advertised on Russian-language dark web forums and used by cybercriminals throughout the former Soviet Union. The author, Profexer, is a well-regarded technical expert among hackers, spoken about with awe and respect in Kiev.

Serhiy Demediuk, chief of the Ukrainian Cyber Police, said in an interview that Profexer went to authorities himself. As the cooperation began, Profexer went dark on hacker forums. He last posted online on Jan. 9. Demediuk said he had made the witness available to the FBI, which has posted a full-time cybersecurity expert in Kiev as one of four bureau agents stationed at the U.S. Embassy there. The FBI declined to comment.

Profexer was not arrested because his activities fell in a legal gray zone, as an author but not a user of malware, the Ukrainian police say. But he did know the users, at least by their online handles. “He told us he didn’t create it to be used in the way it was,” Demediuk said.

A bear’s lair

While it is not known what Profexer has told Ukrainian investigators and the FBI about Russia’s hacking efforts, evidence emanating from Ukraine has again provided some of the clearest pictures yet about Fancy Bear, or Advanced Persistent Threat 28, which is run by the GRU.

Fancy Bear has been identified mostly by what it does, not by who does it. One of its recurring features has been the theft of e-mails and its close collaboration with the Russian state news media.

Tracking the bear to its lair, however, has so far proved impossible, not least because many experts believe that no such single place exists.

Even for a sophisticated tech company like Microsoft, singling out individuals in the digital miasma has proved just about impossible. To curtail the damage to clients’ operating systems, the company filed a complaint against Fancy Bear last year with the U.S. District Court for the Eastern District of Virginia but found itself boxing with shadows.

As Microsoft lawyers reported to the court, “because defendants used fake contact information, anonymous Bitcoin and prepaid credit cards and false identities, and sophisticated technical means to conceal their identities, when setting up and using the relevant internet domains, defendants’ true identities remain unknown.”

Nevertheless, Ukrainian officials, though wary of upsetting the Trump administration, have been quietly cooperating with U.S. investigators to try to figure out who stands behind all the disguises.

Included in this sharing of information were copies of the server hard drives of Ukraine’s Central Election Commission, which were targeted during a presidential election in May 2014. That the FBI had obtained evidence of this earlier, Russian-linked electoral hack has not been previously reported.

Traces of the same malicious code, this time a program called Sofacy, were seen in the 2014 attack in Ukraine and later in the DNC intrusion in the United States.

Intriguingly, in the cyberattack during the Ukrainian election, what appears to have been a bungle by Channel 1, a Russian state television station, inadvertently implicated government authorities in Moscow.

Hackers had loaded onto a Ukrainian election commission server a graphic mimicking the page for displaying results. This phony page showed a shocker of an outcome: an election win for a fiercely anti-Russian, ultraright candidate, Dmytro Yarosh. Yarosh in reality received less than 1 percent of the vote.

The false result would have played into a Russian propaganda narrative that Ukraine today is ruled by hard-right, even fascist, figures.

The fake image was programmed to display when polls closed at 8 p.m. but a Ukrainian cybersecurity company, InfoSafe, discovered it just minutes earlier and unplugged the server.

State television in Russia nevertheless reported that Yarosh had won and broadcast the fake graphic, citing the election commission’s website, even though the image had never appeared there. The hacker had clearly provided Channel 1 with the same image in advance, but the reporters had failed to check that the hack actually worked.

“For me, this is an obvious link between the hackers and Russian officials,” said Victor Zhora, director of InfoSafe, the cybersecurity company that first found the fake graphic.

A Ukrainian government researcher who studied the hack, Nikolai Koval, published his findings in a 2015 book, “Cyberwar in Perspective,” and identified the Sofacy malware on the server.

The mirror of the hard drive went to the FBI, which had this forensic sample when the cybersecurity company CrowdStrike identified the same malware two years later, on the DNC servers.

“It was the first strike,” Zhora said of the earlier hack of Ukraine’s electoral computers. Ukraine’s Cyber Police have also provided the FBI with copies of server hard drives showing the possible origins of some phishing e-mails targeting the Democratic Party during the election.