Jonathan Kirsch is the author of, most recently, “The Short, Strange Life of Herschel Grynszpan.”

‘To tell the truth, I used to read and write German,” a 73-year-old retired potato chip salesman named Jack told a federal prosecutor at the outset of his interrogation in 1992. “Now I have forgotten.”

Jack, as we discover in “Citizen 865: The Hunt for Hitler’s Hidden Soldiers in America” by Debbie Cenziper, was actually Jakob Reimer, one of the Nazi mass murderers trained in a village in southeastern Poland called Trawniki. The author calls it “one of the most diabolical operations in the Holocaust,” but “the Trawniki men,” too, are mostly forgotten nowadays.

Cenziper’s mission in “Citizen 865” is to restore and preserve our memory of Trawniki through her account. She is a Pulitzer Prize-winning journalist on The Washington Post’s investigative team and director of investigative journalism at Northwestern University’s Medill School of Journalism; she’s based at Medill’s Washington campus. Cenziper brought her investigative skills to bear on the challenge of retrieving the hard facts, but she also possesses the gift of a storyteller. For that reason, “Citizen 865” is a work of nonfiction that reads like a thriller.

She shows us the human faces of the real men and women of her narrative — the victims, the survivors, the perpetrators and those who sought to bring the perpetrators to justice. Prominent among them are the lawyers, investigators and historians who refused to allow the victims to disappear into mass graves. The Office of Special Investigations (OSI), a unit of the Justice Department, took up the task in 1978, when Rep. Elizabeth Holtzman of Brooklyn “helped pass legislation that made it easier for the federal government to deport anyone found to have participated in Nazi persecution, striking down a series of exemptions long provided under immigration law.”

The Nazi-hunters included historians from Brandeis and Columbia; “classic gun-and-badge” investigators from the U.S. Fish and Wildlife Service, the IRS and the U.S. Marshals Service; and court-hardened criminal prosecutors. Their goal was to revoke the citizenship of Nazi war criminals who had lied their way into the United States three decades earlier: “If American citizenship is to have any meaning at all,” observed historian David Marwell, “we have to take it back.”

The OSI pursued dozens of Nazi war criminals in the United States, including the notorious John Demjanjuk, and Cenziper shows us both the successes and the failures. (Two different prosecutions were required to strip Demjanjuk of his citizenship.) But it is Jakob Reimer, a member of “the Trawniki elite” whose identification number was 865, who is placed in the crosshairs of her book.

A descendant of immigrants from Germany to Russia, Reimer was born in a Mennonite community in Ukraine, then a part of the Soviet Union, in 1918. He was serving as an officer in the Red Army when he was taken prisoner by German soldiers in 1941, and the SS plucked him out of a POW camp for training and service at Trawniki. Reimer insisted that he was only a civilian paymaster, but in fact he rose to the SS rank of top sergeant and was “deployed on at least two critical missions [that sent] forty thousand Jews to their deaths in the Treblinka gas chambers” in 1942, as well as the crushing of the Warsaw Ghetto uprising and the final liquidation of the Jews confined there in 1943.

Reimer’s war record was overlooked when he was granted entry to the United States and, later, full citizenship, but the author allows us to witness the interrogation that finally revealed the truth in 1992. “Reimer had always excelled, as a Red Army lieutenant, as a commander at Trawniki, as an immigrant, as an American,” Cenziper writes, echoing the thoughts of his interrogator. “And somewhere deep in the woods outside Trawniki, Reimer had excelled as a participant in a mass-killing operation.”

The decision that finally revoked Reimer’s citizenship in 2002 was hailed by James Comey, then serving as U.S. attorney in New York: “Reimer’s presence in the United States is an affront to all those killed in the Holocaust.” His appeal was rejected by the U.S. Court of Appeals for the 2nd Circuit in an opinion written by future Supreme Court justice Sonia Sotomayor. But Germany refused to accept him back, and he died in the United States before the OSI and the State Department “could find a country willing to take him.”

No matter how many times we peer into the black hole of the Holocaust, something new and strange is there to behold. “At Belzec,” Cenziper writes about the pathway from the barracks to the gas chamber, “signs in German, Polish, and Yiddish read: Please undress. Go to the bath area. Shower. Relax on the straw.” Later, she quotes a remark by Eli Rosenbaum, director of the OSI, about other “late-night images” from the historical record that have the power to haunt us: “You try to imagine these things,” Rosenbaum would say, “and then you try real hard not to imagine them.”

Rosenbaum’s words remind us of Cenziper’s greatest accomplishment in a highly significant work of investigation that is eye-opening and heartbreaking. She compels us to confront the crimes of the Trawniki men in a way that burns itself into both memory and history.

Citizen 865

The Hunt for Hitler's Hidden Soldiers in America

By Debbie Cenziper

Hachette. 300 pp. $28