SEOUL ¨C Many South Korean workers were sent to the U.S. on questionable documents despite their misgivings and warnings about stricter U.S. immigration enforcement before last week¡¯s raid on a Hyundai site, according to workers, officials and lawyers.

For years, South Korean companies have said they struggle to obtain short-term work visas for specialists needed in their high-tech plants in the United States, and had come to rely on a grey zone of looser interpretation of visa rules under previous American administrations.

When that changed in the early days of U.S. President Donald Trump¡¯s second term, some workers were denied entry to the United States under statuses that did not fully allow work, according to interviews with more than a dozen workers from various companies, government and company officials, and immigration lawyers.

More than 300 South Koreans were among the 475 people swept up and detained by U.S. federal authorities at Hyundai Motor¡¯s car battery plant near Savannah, Georgia, on Thursday, in the largest single-site enforcement operation in the history of the Department of Homeland Security¡¯s investigative operations.

Many of the people arrested were skilled workers who were sent to the U.S. to install equipment at the near-complete factory on a visa waver program, or B-1 business traveler visas, which largely did not allow work, three people said.

¡°It¡¯s extremely difficult to get an H-1B visa, which is needed for the battery engineers. That¡¯s why some people got B-1 visas or ESTA,¡± said Park Tae-sung, vice chairman of Korea Battery Industry Association, referring to the Electronic System for Travel Authorization.

One person who works at the Georgia site said that this had long been a routine practice. ¡°There was a red flag. ... They bypass the law and come to work,¡± the person said, asking not to be named because of the sensitivity of the matter.

The arrests shocked South Korea although workers had previously expressed concern that they could be caught in between Trump¡¯s immigration crackdown and corporate efforts to protect investments in the United States that are at the center of ongoing trade and tariff talks.

An equipment technician in South Korea, who previously worked with six of the people arrested, said: ¡°I warned them they could screw up their lives if they are caught.¡±

¡°I begged them not to go to the United States again,¡± he said, speaking on condition of anonymity.

He said he had once obtained a B-1 visa from the United States by claiming he was a supervisor, rather than an equipment specialist.

Another equipment technician working as a contractor with LG Energy Solution said his application for a B-1 visa to work at Hyundai¡¯s Georgia factory was rejected earlier this year, without...