President Donald Trump’s pardon of former Arizona sheriff Joe Arpaio amounts to an endorsement of the idea of concentration camps, says a journalist who has reported on the global history of the deadly facilities.

Arpaio referred to his own county jail as a “concentration camp.” For over two decades, he operated “Tent City,” where detainees were kept in brutal conditions, including temperatures soaring well above 100 degrees Farenheit. They were also forced to work on chain gangs and to wear pink undergarments as a form of humiliation. Arpaio was convicted in July of criminal contempt for ignoring a court order prohibiting the detention of people based on mere suspicions about their legal status.

In an email to HuffPost Saturday, Andrea Pitzer, the author of One Long Night: A Global History of Concentration Camps, defined a concentration camp as a “mass civilian detention outside the standard legal process, usually on the basis of race, ethnicity, or political activity.” While Pitzer said Tent City was a prison technically constructed to hold those convicted by law, it bore familiar elements to a concentration camp, including “brutal dehuminization.”

“Once Arpaio began neighborhood sweeps and traffic stops deliberately targeting Latinos, and then detaining them without charges, his whole enterprise tilted further toward being a concentration camp for that set of detainees,” she wrote. “And even for those who had been convicted of crimes, it was a harrowing, often deadly experience.”

Pitzer said Trump pardoning Arpaio legitimized the 85-year-old former sheriff’s operation.

“When it was just Arpaio and his deputies doing it, it was a freelance, loose-cannon operation. What happened yesterday is that the President of the United States put his position behind it and used executive power to bless these tactics,” she wrote. “Historically, when this kind of thing has happened, it’s encouraged other people to take up the same tactics. I think we need to hear from the Department of Justice whether official guidance is forthcoming about the use of these strategies by law enforcement.”

The White House said Friday Arpaio deserved a pardon because of his history in law enforcement. Trump has faced widespread criticism for his decision to pardon Arpaio, including from Sen. John McCain (R-Ariz.).