(CNN) Donald Trump's grandfather was kicked out of his native Germany for failing to do his mandatory military service there, a historian has claimed.
A local council letter from 1905 informed Friedrich Trump -- who had become a United States citizen -- that he would not be granted his German citizenship back and that he had eight weeks to leave the country or be deported, German historian Roland Paul told CNN Tuesday.
He also claimed that Trump had illegally left Germany, failing to notify authorities of his plan to immigrate.
The Trump camp did not immediately respond to CNN's questions on the research.
The findings have sparked interest as President-elect Trump vowed to crack down on illegal immigration in his election campaign.
"Trump talks against illegal immigration, so I think he should remember his own family story from time to time," Paul said.
Trump proposed a wall be built between the US and Mexico as the centerpiece of his anti-immigration drive. He has also threatened to deport undocumented migrants who have committed crimes in the US.
Paul came across the document on Friedrich Trump's threatened deportation in state archives and also found several letters from him pleading with authorities to allow him to stay. Paul mused on how this one administrative decision seemed to have changed the course of history.
"I have to say this kind of letter wasn't unusual back then," Paul told CNN.
"But if they had accepted him back at that time in 1905, Donald Trump wouldn't be in the US -- in fact, he wouldn't exist."
He said the eight week deadline was extended as Friedrich Trump said that his daughter was ill. But he was finally forced to leave two months later.
Trump's father was born in the US, where he met Trump's mother, Mary Anne MacLeod, who hailed from Scotland.
Paul, now retired, carried out his research earlier this year when he was Director of the Institute for Palatine History and Folklore in Kaiserslautern, but his research caught public attention on Monday when the tabloid Bild published his findings.