KYIV — When Oleksandr Repetylo arrived in July 2017 at Izolyatsia — a prison in eastern Ukraine’s separatist region — his head was covered by a sack and he learned how to distinguish his captors by their voices.

Repetylo was among thousands of people detained by rebels or Ukrainian government forces during the ongoing battles between Kyiv’s Western-allied military and Moscow-backed separatists — a nearly eight-year-old conflict that has killed close to 14,000 people.

But Repetylo’s time in custody was spent at one of the most infamous sites in the breakaway east. Former inmates have for years described torture and other horrors and humiliations.

Now, after the arrest of Izolyatsia’s suspected former commandant, more details could emerge on what went on inside the prison’s walls. It could also potentially hand Ukrainian investigators more evidence against other suspected Izolyatsia guards and boost a current case at the European Court of Human Rights filed by former Izolyatsia prisoners.

“He would be just walking by, open the door, and beat you — no reason, nothing, just came in and beat you,” said Repetylo, who was accused by rebels of aiding the Kyiv government — a charge that he denied.

“Everyone waited for the night,” he said in a phone interview. “Night was horrible.”

Repetylo, who said he was often hooded by his captors, said that at first he distinguished the torturer by his voice, which was sometimes noticeably drunk. One of his cellmates died of the beatings, Repetylo said.

With time, Repetylo learned the name: Denys Pavlovych Kulykovsky — better known by his nom de guerre “Palych.”

On Nov. 9, Ukrainian authorities announced that they had apprehended Kulykovsky and placed him in two months pretrial detention. The suspect has been charged with four counts, including “violating the laws and customs of war.” No trial date has been set.

The suspect’s lawyer for his pretrial hearing, Nadiya Albermakh, did not comment on the charges or the alleged background of her client. She acknowledged that the suspect had been living in Kyiv for two years.

The news that officials had captured one of the war’s most-wanted figures created a sensation in Ukraine. A possible trial could help shed light on a prison camp shrouded in secrecy as well as on some of the darkest corners of the country’s ongoing war.

Yet, it also underscores the difficulties Ukraine could face in achieving reconciliation if a peace deal is ever reached.

Videos released after the suspect’s arrest showed a man whose face was obscured or partially covered by a surgical mask.

But Repetylo knew it was him. The voice gave him away, he said.

“Yes, that’s his voice, that’s his voice,” he said. “It’s impossible to forget.”

Repetylo, who had a transportation company, spent almost 10 months at Izolyatsia and then was transferred to another facility in the breakaway east. In December 2019, he was freed in a prisoner exchange between Kyiv and rebels and lives in government-controlled Ukraine.

The separatist authorities in eastern Ukraine claim that Izolyatsia, meaning “Isolation,” does not exist.

But Western researchers, officials and former prisoners say the facility is a centerpiece of the separatists’ penal network, a “Donetsk Dachau” in the words of a former inmate, referring to a part of the rebel-held east.

The prison is a warren of tunnels, rooms and underground shelters on the grounds of an abandoned electrical supplies factory, according to accounts from former prisoners. Before the war, a local artists’ collective converted the space into a center for exhibitions.

A report on abuses in eastern Ukraine, released this year by the U.N. Office of the High Commissioner for Human Rights, said that “torture and ill-treatment were carried out systematically” at Izolyatsia and other separatist detention centers.

The United Nations said that government forces and the insurgents have conducted “arbritary detention, torture and ill-treatment,” although Ukrainian forces are said to have committed their abuses largely in the beginning of the war.

Officials in Kyiv have been slow, however, to investigate crimes reportedly committed by pro-government forces, the report said.

The abuses at Izolyatsia were unique in their cruelty, former prisoners say, with the man known as Palych often taking the lead.

“A psychopath, a sadist,” said Stanislav Aseyev, a journalist who was held for 28 months at Izolyatsia and wrote a book about his experiences, “The Torture Camp on Paradise Street.” (The title is a play on Izolyatsia’s address: 3 Bright Way St. in the city of Donetsk.)

In addition to the beatings, inmates alleged that Palych would subject them to rape, sexual humiliation, electric-shock torture, mock executions and other brutalities. Sometimes, the abuse was carried out with the cooperation of Russia’s security services, the former inmates allege.

Moscow denies any involvement in the separatist regions. But a report released this month by the group Conflict Armament Research found that Russia has regularly supplied the insurgents with arms. The findings were based in part on tracing serial numbers and other identifying markings.

The events leading up to the suspect’s arrest remain unclear.

Aseyev, the journalist, said he and members of the Netherlands-based investigative outlet Bellingcat conducted their own search for Palych and discovered he had been living in Ukraine’s capital, Kyiv, for more than two years.

The head of Ukraine’s security services, Ivan Bakanov, told reporters that the suspect was arrested in the capital. Bakanov did not provide further details, saying the investigation was ongoing.

The security services, known as the SBU, released a video in which the suspect states he was Izolyatsia’s commandant for a period and tortured detainees to extract confessions. He also named alleged members of Russia’s Federal Security Service who he said were involved. No further details were given on how the purported confession was obtained or on other aspects of the investigation. The suspect’s name was not given in the SBU video and his face was obscured.

The defense lawyer Albermakh said she could not comment on the SBU video, which was made before she was assigned to her client.

“[Palych] is the most important witness who can say, ‘Yes, [Izolyatsia] existed, and I headed it, and Russia did it,’ ” said Oleh Kulakov, another former inmate. “He’s very valuable, and he needs to be protected.”

For Repetylo, the arrest provoked a “storm of emotions.”

“Of course, joy,” he added.

For others, the overwhelming feeling was rage.

Kulakov fantasizes about himself and former inmates turning against their former torturer — even though he makes clear he has no intention of carrying out reprisals.

“When I would do this, I wouldn’t be thinking of how he beat me, but for those who had it worse than me,” he said. “I would be taking revenge for these people.”