For months after the Russian military occupied his hometown in southern Ukraine, Yevheny made plans to leave. But each time, the 48-year-old lawyer said, he was thwarted, by warnings of gunfire on the roads and, once, when a hired driver disappeared.

Then came the Russian soldiers, who searched his house, hauled him off to a nearby village and cast him into a dark cellar where he would undergo a violent, weeklong interrogation.

Yevheny’s harsh treatment is just one example of a colonialist repression Russia is enforcing across the Ukrainian territory it controls, a system comprising a gulag of more than 100 prisons, detention facilities, informal camps and basements that is reminiscent of the worst Soviet excesses.

Research by a team of reporters involving dozens of interviews with former detainees, human rights organizations and Ukrainian officials from the Office of the General Prosecutor, the intelligence service and ombudsmen, reveals a highly institutionalized, bureaucratic and frequently brutal system of repression run by Moscow to pacify an area of 40,000 square miles in Ukraine, roughly the size of Ohio.

The abuses almost always occur unseen and unheard by the outside world, as Russia-controlled areas are largely inaccessible to independent journalists and human rights investigators. But human rights organizations and Ukrainian prosecutors and government officials have managed to monitor the situation closely, drawing on accounts from civilians who are either still living there or who have found a way to leave.

The ultimate aim of Moscow’s efforts, rights advocates said, is to extinguish Ukrainian identity through such tactics as propaganda, re-education, torture, forced Russian citizenship and sending children to live in Russia.