There is little respite from victory for Ukrainian forces in newly liberated Kupyansk.  Russian shells still pound its streets, marring the horizon with black smoke.   

  Severe damage is visible in almost every building.  A huge billboard with an image of a waving Russian flag stands next to the bridge that crosses the Oskil River in the center of the city and bears the words, “We are one people with Russia!”   

  For now, the Ukrainian military has chased Russian forces over the bridge and appears to be building some momentum pushing up the eastern banks of the river toward Luhansk, a key separatist region controlled by Moscow.  CNN saw Ukrainian infantry returning from the east side on foot.   

  Yet within this city, one of several in the eastern Kharkiv region that have been liberated, there are the telltale signs of a hellish occupation.  A former police building was used as a massive detention center by the Russians, at one point holding up to 400 prisoners in its cramped and dark cells, with eight or nine prisoners per room, Ukrainian authorities told CNN.  A painted mural of a Russian soldier with a “Z” on his armband standing next to an old woman waving the flag of the former Soviet empire is still visible on one wall.   

  Before CNN was allowed inside, a detainee with his hands bound in shiny blue tape was quickly ushered out, placed in a vehicle and driven away.   

  This was possibly a Russian soldier, according to the Security Service of Ukraine (SBU), who is believed to have either deserted his troops or stayed behind.  The prisoner claimed to be local, the SBU said.   

  Just outside the building’s entrance, two Russian flags tied to wooden poles were strewn on the floor, one with burn marks.  Inside, trash littered the wet room floors.  Through the narrow corridor were small rooms on each side, where the Russians used to keep their prisoners.   

  A few small mattresses and tables were seen in some of the small cells, others held only a table and two chairs, remnants of what might have been an interrogation room.   

  Not all rooms had been cleared of possible explosives, officials told CNN.  A grenade trap sat on a bench inside a cell, held in place by a half-eaten can.   

  As CNN drove through the center, an SBU officer spotted the trap and wrote “Grenade!!!”  on the wall outside the cell with a black marker and an arrow to indicate exactly which room the investigators should enter.  The door was closed.   

  As investigations continue, Ukrainian officials are discovering other scars, such as those from alleged torture.   

  A former detainee introduced to CNN by Ukrainian security services said he was imprisoned in the building about a month ago.  Walking down the hall, he showed CNN the room where he said the Russians interrogated him.   

  “They put me in this chair,” said the ex-prisoner – whom CNN did not identify for his safety – pointing.  “There sat the investigator and there was the guy with the phone and another guy who helped.”   

  The phone was an old wind up model that he said was used to give him electric shocks.  He thinks his interrogator was familiar with this method from their time in the Russian Security Services, the FSB.   

  The captors asked him who he had contact with from the Ukrainian army and he told them he was once a cook in the army, he said.   

  “They said to me, ‘You think you’re tough.  Let’s find out how tough,” he said.  “They also shot me with a pistol.  Here and on the leg,” he told CNN, pointing to his chest and leg.   

  “They promised that I would only see the sun and the sky again if they put me in a minefield,” he said.  “The main thing is to survive and endure.  It took me a week and a half to recover when I got out.”   

  The man is not the only one dealing with the scars of a brutal invasion, detention and alleged torture.   

  As authorities continue to search and clear liberated cities in the Kharkiv region, they are finding more and more evidence of detention centers and cells used for torture.   

  Ukrainian President Volodymyr Zelensky said on Saturday that “more than 10 torture chambers” used by the occupation forces have so far been found in the region.  “As the occupiers fled, they also dropped their torture devices,” he said.   

  CNN reached out to the Russian government for comment, but did not receive a response.   

  Kupiansk may have been liberated recently, but the city is a ghost town, marked by destruction and debris.   

  The few locals who remain huddle in its empty shell.