The news on Monday that Poland will request the extradition of 98-year-old Michael Karkoc, who is believed by authorities to have been a commander of a Nazi-led unit that was responsible for war crimes during World War II, is the latest development to arise from years of questioning about the Minnesota man’s past.

It is also a fresh reminder that the hunt to find and prosecute the perpetrators of the war crimes of that era, the period that gave the world the word genocide, is ongoing — including within the United States.

When Karkoc’s case came to light in 2013, Rabbi Marvin Hier of the Simon Wiesenthal Center told New York magazine that there could be “hundreds” of Nazi war criminals currently living, undetected, in the United States. (Karkoc’s family, meanwhile, insists that he is not one of them.) Per a 2008 report from the Office of Special Investigations (OSI), a bureau TIME once described as “Nazi-hunting branch” of the Justice Department, in the quarter-century after its founding in 1979 its work had already led to 83 war criminals losing their naturalized U.S. citizenship, 62 leaving the U.S. and about 200 being barred from entry.

Unsurprisingly, one of the substantial obstacles faced by the OSI was that more than 30 years had passed between the end of the World War II and the establishment of the office. People had moved, names had been changed, faces had aged, documents had been lost or destroyed, memories had faded. In the immediate aftermath of the war, the push to prosecute Nazis had been a major world movement — so what had happened to create a decades-long gap in the U.S.?

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According to a recent report from the agency itself, which became public in 2010, several incidents in the 1970s — such as the much-publicized discovery that an infamous Majdanek concentration camp guard was living a quiet life in Queens, N.Y. — led the American public to demand that the government change the way it did business in this area, leading to the formation of the OSI. (The OSI later merged with another department to become the Human Rights and Special Prosecutions Section.) Though the office was originally intended to investigate those who committed war crimes and other violations on that level, not to investigate the U.S., it naturally ended up facing questions of how those war criminals had gotten to the U.S. in the first place and why it took so long to go after them.

The answer, as the report found, was that the presence of war criminals in the United States was no accident:

OSI did not originally conceive its mission as including the need to answer these questions. But it was inexorably drawn to the issues when subjects argued that they were in the country at the behest, or with the knowledge, of the United States — allegedly in return for information or services supplies to the government during or after the war. OSI learned that some persecutors were indeed knowingly granted entry. America, which prided itself on being a safe haven for the persecuted, became — in some small measure — a safe haven for persecutors as well. Some many view the government’s collaboration with persecutors as a Faustian bargain. Others will see it as a reasonable moral compromise borne of necessity.

In essence, the U.S. admitted that, during the Cold War, war criminals were allowed immigrate if they were “deemed useful by Washington in the struggle against the old Soviet Union,” as TIME put it when the report came out. Some were even specifically recruited by various agencies for the intelligence or research knowledge they could bring.

Moreover, even when war criminals were not knowingly brought to the U.S. for Cold War purposes, the immigration system in the postwar years was set up in a way that allowed them to relatively easily slip in. Facing a huge number of refugees and a world in which that concept was only just being defined, the immigration system, with its non-refugee quotas that favored Western Europeans and its mandate to focus on helping people fleeing communism, was not equipped to deal with the issue. As the OSI reported, in the interest of speed laws were passed that in effect acknowledged that the U.S. couldn’t keep out those who had been enemies, though the government reserved the right to send them away later.

Several cases of Nazi trials in Germany have made news recently, particularly in 2016, at which point Eliza Gray explored the factors leading to that confluence in a story for TIME. As she explained, for decades the German (and previously West German) penal code was “effectively treating deaths in the Holocaust like any other murder,” meaning that a guilty verdict required proof of personal culpability in a specific killing. Though investigations continued, those who argued that “they had only been following orders” often got off in Germany, though in other countries “perhaps 1,000 camp personnel faced trials.” In recent years, however, German law has changed. After the 2011 conviction of John Demjanjuk, in which it was held that he had come to the U.S. after serving as a guard at Sobibor, prosecutors saw that they could successfully go after those who had participated in the Holocaust for facilitating death, even if their personally committing a particular murder could not be proved. That legal change has matched a more widespread evolution of thinking about genocide, by which the idea that “following orders” is essentially an excuse has faded.

Different justice systems have different standards as to what is required in order to prosecute and convict in such cases. (For example, though Poland seeks Karkoc’s arrest German authorities investigated him too, the Washington Post reports, but determined that due to his health he would be unable to stand trial anyway.) But the sense of urgency in the matter is bigger than any one justice system.

Though the Washington Post estimated in 2014 that, given the age someone would have had to have been at the time and average life expectancy, the hunt might continue into the 2040s, U.S. authorities — including historians — as well as those in other countries know that the time is running out to find and prosecute these people.

As the number of Holocaust survivors and witnesses dwindles — it was estimated in 2016 only 100,000 survivors are still alive — the urgency of the cases for which they are crucial witnesses increases, and of course the criminals’ aging lends its own kind of rush to the situation.

“Some say that it’s too late to hunt the criminals down,” Robert Janicki of the Commission for the Prosecution of Crimes Against the Polish Nation told the New York Times in response to the news about Karkoc, “but I don’t think it is.”

Write to Lily Rothman at lily.rothman@time.com.