Thousands of children have been found in the basements of war-torn cities like Mariupol and at orphanages in the Russian-backed separatist territories of Donbas. They include those whose parents were killed by Russian shelling as well as others in institutions or with foster families, known as “children of the state.”

Russia claims that these children don’t have parents or guardians to look after them, or that they can’t be reached. But the AP found that officials have deported Ukrainian children to Russia or Russian-held territories without consent, lied to them that they weren’t wanted by their parents, used them for propaganda, and given them Russian families and citizenship.

The investigation is the most extensive to date on the grab of Ukrainian children, and the first to follow the process all the way to those already growing up in Russia. The AP drew from dozens of interviews with parents, children and officials in both Ukraine and Russia; emails and letters; Russian documents and Russian state media.

Whether or not they have parents, raising the children of war in another country or culture can be a marker of genocide, an attempt to erase the very identity of an enemy nation.

Even where parents are dead, Rapp said, their children must be sheltered, fostered or adopted in Ukraine rather than deported to Russia.

Russian law prohibits the adoption of foreign children. But in May, Putin signed a decree making it easier for Russia to adopt and give citizenship to Ukrainian children without parental care — and harder for Ukraine and surviving relatives to win them back.

Russia also has prepared a register of suitable Russian families for Ukrainian children, and pays them for each child who gets citizenship — up to $1,000 for those with disabilities. It holds summer camps for Ukrainian orphans, offers “patriotic education” classes and even runs a hotline to pair Russian families with children from Donbas.

“It is absolutely a terrible story,” said Petro Andryushchenko, an adviser to the Mariupol mayor, who claims hundreds of children were taken from that city alone. “We don’t know if our children have an official parent or (stepparents) or something else because they are forcibly disappeared by Russian troops.”

Russia portrays its adoption of Ukrainian children as an act of generosity that gives new homes and medical resources to helpless minors. Russian state media shows local officials hugging and kissing them and handing them Russian passports.

It’s very hard to pin down the exact number of Ukrainian children deported to Russia — Ukrainian officials claim nearly 8,000. Russia hasn’t given an overall number, but officials regularly announce the arrival of Ukrainian orphans in Russian military planes.

In March, Russian children’s rights ombudswoman Maria Lvova-Belova said more than 1,000 children from Ukraine were in Russia. Over the summer, she said 120 Russian families had applied for guardianship, and more than 130 Ukrainian children had received Russian citizenship. Many more have come since, including a batch of 234 in early October.

She acknowledged that at first, a group of 30 children brought to Russia from the basements of Mariupol defiantly sang the Ukrainian national anthem and shouted, “Glory to Ukraine!” But now, she said, their criticism has been “transformed into a love for Russia,” and she herself has taken one in, a teenager.

The children of Mariupol aren’t the first Russia has been accused of stealing from Ukraine.

In 2014, after Russia annexed the Crimean Peninsula, more than 80 children from Luhansk were stopped at checkpoints and abducted. Ukraine sued, and the European Court of Human Rights found the children were taken into Russia “without medical support or the necessary paperwork.” The children were returned to Ukraine before a final decision.

Kateryna Rashevska, a human rights defender, said she knows of about 30 Ukrainian children from Crimea adopted by Russians under a program known as Train of Hope. Now, she said, some of those children might well be Russian soldiers. Since 2015, the Young Army Cadets national movement has trained youth in Crimea and Russia for potential recruitment into the military.

This time around, at least 96 children have been returned to Ukraine since March after negotiations. But Ukrainian officials have tracked down the identities of thousands more in Russia, and the names of many others simply aren’t published.