Twenty people have been charged for their roles in a multimillion-dollar birth-tourism business, authorities said Thursday, marking the largest-ever crackdown on businesses helping Chinese women travel to the U.S. to give birth. On Thursday, three people were arrested in Southern California on charges including conspiracy, visa fraud, and money laundering, according to grand-jury indictments unsealed Thursday in Federal District Court in Los Angeles. More than a dozen others have also been charged in similar cases. For thousands of dollars, the businesses allegedly brought hundreds of pregnant Chinese women to California so their children would automatically receive U.S. citizenship. It is the first time the United States has criminally prosecuted birth-tourism operators, authorities said, as many of the women hid their pregnancies and lied about details of their trips to throw off investigations.

“I see this as a grave national security concern and vulnerability,” said Mark Zito, assistant special agent-in-charge of Immigration and Customs Enforcement’s homeland security investigations in Los Angeles. “Are some of them doing it for security because the United States is more stable? Absolutely. But will those governments take advantage of this? Yes, they will.”