When Youngjin looked out of his office window and saw armored trucks and immigration enforcement officers running around with guns, he was surprised, but not worried. The young South Korean was certain it had nothing to do with him. He was in the U.S. only for a few weeks on a short-term visa, he thought to himself.

Then the armed agents burst into his room and ordered him outside. They handcuffed him, before attaching chains to his waist and ankles and loaded him onto a bus bound for a detention center. I panicked and my mind went blank. I felt sick, he told the BBC, now back home in South Korea. I couldn't understand why I was being treated like this.

Youngjin is one of more than 300 Korean workers who were detained in the U.S. state of Georgia earlier this month, in one of the largest immigration raids of Donald Trump's presidency to date. He and others interviewed for this article didn't want to reveal their real names in order to protect their identity.

U.S. officials initially claimed the workers had been in the country illegally on incorrect visas, but eventually, the two sides struck a deal allowing them to leave voluntarily without any penalties, so they could return to work there in the future. Most of the workers were in the U.S. temporarily, helping to build an electric car battery plant run by two South Korean companies, Hyundai and LG – part of a U.S. push to get foreign companies to invest and manufacture more in the states.

LG said that many of its employees who were arrested had various types of visas or were under a visa waiver program. And so they were especially shocked by the raid.

We just came out for a brief break and I could see a lot of people, officials with guns. As Koreans, we just thought they were here to arrest criminals, but then, they suddenly started arresting us, said Chul-yong, who was also detained that day.

He said they tried to explain who they were, but they were terrified: There were helicopters and drones, armored vehicles… people with guns. There were some officers pointing guns at workers, he claimed. You know those red lasers that come out of those guns? It was so shocking that some people were shivering in fear.

Even those who managed to share their visa details said they were arrested. I thought everything would be cleared up but instead, they suddenly shackled us, said Mr. Kim, another worker detained, who only wanted to reveal his last name.

Youngjin, an engineer and sub-contractor for LG, was scheduled to be there for five weeks to train staff to operate some of the specialist high-tech equipment.

The 30-year-old was shaking, still visibly upset, as he described to the BBC being taken to the detention center and locked in a room with 60-70 other people.

I had a panic attack. I just stood there trembling, he said. The room was freezing and the new detainees were not given blankets for the first two days, he added.

The worst part was the water. It smelt like sewage. We drank as little as possible.

He only leaves the house for short outings. When I'm outside, if I smell something similar to the prison, I start trembling and get short of breath, so I don't go outside for long now, he said.

And seeing himself in news reports on TV has not been easy. You couldn't see my face, but you could recognize my body. So my family and friends all knew that it was me.

He thinks most of the workers have had enough and may not return. But he says he has no choice. This is what I do. I've been doing this for 30 years. I've put my life into this work, he adds.