Rep. Alexandria Ocasio-Cortez’s (D-N.Y.) controversial comments likening U.S. immigrant detention centers at the southern border to “concentration camps” aren’t inaccurate, a concentration camp expert and author said Tuesday.

Andrea Pitzer, who literally wrote the book on the global history of concentration camps, defended the freshman lawmaker against Republican backlash during an appearance Tuesday on MSNBC’s “All In With Chris Hayes.”

What the U.S. is doing at the U.S.-Mexico border today by detaining hundreds of thousands of undocumented immigrants and children “fits very cleanly” inside the historical definition of concentration camps, Pitzer argued.

Such barbaric facilities existed decades before the creation of Auschwitz and other Nazi Germany political prisons, where millions of Jewish people were murdered in the Holocaust during World War II, she noted.

Though the term “concentration camp” today often elicits images of Holocaust death camps, it’s also been used historically to refer to the mass detention of people, often political prisoners or members of a persecuted minority group, without trial.

“The death camps ― which were on top of the existing concentration camps system, including Auschwitz and a series of other camps in which you had gas chambers, mass killings ― that is a singular moment in history,” Pitzer said Tuesday. “And for the people that want to respect that, I think that’s fine and that’s important.”

She continued: “If we want to call it ‘irregular detention,’ if we want to call it ‘extrajudicial detention,’ I don’t think we have to get stuck on that term. I wrote a history of the term ... and what I would really like people to know is that the same thing is happening here now.”

Ocasio-Cortez faced intense outrage this week from some congressional Republicans after sharing an Esquire article on Twitter that cited Pitzer to draw comparisons between concentration camps and immigrant detention centers at the U.S. southern border.

President Donald Trump’s administration “has established concentration camps on the southern border of the United States for immigrants, where they are being brutalized,” Ocasio-Cortez wrote in her tweet linking to the story on Tuesday. “This is not hyperbole. It is the conclusion of expert analysis.”