Growing up in Minnesota, Tyler Graf knew almost nothing about his birth mother. And what little he knew, he said, stung.

His adoption papers listed her name, Hilda del Carmen Quezada; her age, 26; the date, March 2, 1983; and the hospital where she gave birth to him in central Chile. The documents also included a judge’s note saying Ms. Quezada gave him up because she had little money and “other children to support.”

“I never thought that any excuse would be good enough,” said Mr. Graf, who is now a firefighter in Houston. “I carried that animosity, that chip on my shoulder, my whole life.”

The claim that his mother willingly gave him up hurt, Mr. Graf said, until he learned this year that he is one of hundreds — possibly thousands — of Chilean adoptees taken from their parents without their consent during the country’s military dictatorship.

Ms. Quezada, it turned out, had not surrendered her son; she was told the baby, born three months premature, had died.

“Two weeks after the birth, they told me he had died,” Ms. Quezada said. “I asked for the body and they refused, saying it was too small.”

Investigators looking into coercive adoptions in Chile since the first cases came to light in 2014 have come to a stunning conclusion: The practice was widespread during the rule of Gen. Augusto Pinochet, who actively encouraged overseas adoptions to reduce poverty in the 1970s and 1980s. The process was abetted by a vast network of officials — including judges, social workers, health professionals and adoption brokers — who forged documents and are widely assumed to have taken bribes.

More than 550 adoptees have reconnected with their birth families in recent years. But investigators say the scheme, which is still being uncovered, most likely involved many more children.

Judicial officials in Chile are investigating roughly 650 cases of irregular adoptions, a phenomenon the Ministry of Justice called in a statement “extremely serious.”

Mario Carroza, a Chilean judge who opened a criminal inquiry into the adoptions in 2018, said investigators were looking into the circumstances of about 8,000 overseas adoptions that took place from 1970 to 1999. Judge Carroza said the number of cases under scrutiny could reach 20,000.