Croatian prosecutors have opened a war crimes investigation into Jakob Frank Denzinger, an elderly man who allegedly worked as a guard in Nazi concentration camps during World War II.

Denzinger, an ethnic German, currently lives in the eastern Croatian town of Osijek where he was born, according to the Agence France-Presse. The 90-year-old suspect is on a list with several dozen others suspected of Nazi war crimes by special German prosecutors.

When Denzinger was 18 he joined the Waffen SS, a Nazi unit with a particularly brutal reputation, reports Croatian news site Dalje.

After joining the Waffen SS, Denzinger spent several years as a concentration camp guard, working in at least five camps including Auschwitz, Mauthausen, Plaszo, Oranienburg and Buchewals, according to Dalje.

On April 24, 1945 Denzinger was captured by the Allies somewhere in France, but was able to immigrate to the United States in the 1950s, eventually settling in Ohio.

After reinventing himself as a businessman, Denzinger had his citizenship revoked by US District Court Judge John Pratt in 1989, notes the Jewish Telegraphic Agency. Denzinger was denaturalized following a complaint lodged by the local US Attorney’s Office and the Justice Department’s Office of Special Investigations, per the JTA. (The OSI works to root out, denaturalize and deport Nazi war criminals found in the US.)

The next step is for Croatian prosecutors to build a case against Denzinger. Authorities have asked Germany, the US and the Simon Wiesenthal Center to help by sending over any relevant documents or other evidence that might help convict him, notes the AFP.