Standing outside a Russian detention center in Kherson, days after the city was liberated, 29-year-old Ihor still shivered as he recalled what he endured inside.

“I was kept here for 11 days and throughout that time I heard screaming from the basement,” Ihor, who asked CNN not to reveal his last name for his protection, said. “I was stabbed in the legs with a taser, they use it as a welcome. One of them asked what I’d been brought in for and another two of them started hitting me in the ribs.

“People were tortured, they were beaten with sticks in the arms and legs, cattle prods, even hooked up to batteries and electrocuted or waterboarded with water,” he added.

Kherson was the first large city and only regional capital Russian troops have able to occupy since the start of the invasion. Moscow’s armies took over the city on March 2, 2022, and occupied it for several months before being forced to withdraw in early November, after a months-long offensive by Ukrainian forces.

The detention center Ihor was held in was part of a network of at least 20 facilities that Ukrainian and international lawyers said was part of a calculated Russian strategy to extinguish Ukrainian identity.

“These detention centers are linked, they follow a very similar, if not identical way of behaving,” Wayne Jordash, head of the Mobile Justice Team, a collective of international investigators supporting Ukraine’s Office of the Prosecutor General, told CNN.

The investigation found that Russian forces followed a very specific blueprint in several occupied areas, with clear patterns that point to the overarching plan of Moscow’s occupation of Ukraine.

“The first stage, essentially, is to detain and, in many instances, kill a category of people labeled as ‘leaders,’ i.e. those who could physically resist the occupation, but also those who could culturally resist it,” Jordash said.

“The second stage is a sort of filtration process where the population that remains outside of the detention centers is subject to constant monitoring and filtration so that anyone who’s suspected of being involved with ‘leaders’ or been involved with organizing any type of resistance is also then identified and either deported to Russia or detained in the detention centers and tortured.”