IRVINE, Calif. (AP) -- Women who come to the U.S. from China to give birth to automatic American citizens may not be breaking the law by doing so, but it's illegal to lie about why they're coming into the country. That was the clear message federal agents sought to send when they searched three dozen California homes Tuesday in what may have been the biggest federal crackdown yet on so-called maternity tourism. The raids came as homeland security agents investigated three rings catering to such women, who must tell the truth about their visits on visa applications, authorities said. Recent cases in California have catered to wealthy Chinese visitors, most likely due to the country's large population, economic boom and ties to the region. Their numbers are unclear, and no arrests have been made or charges filed. "It is fertile ground for this kind of scheme," said Claude Arnold, special agent in charge for Immigration and Customs Enforcement's homeland security investigations in Los Angeles. "These people were told to lie, how to lie, so that their motives for coming to the U.S. wouldn't be questioned." Shortly after sunrise, dozens of federal agents swarmed an upscale apartment complex in the Orange County city of Irvine, where authorities say a birth tourism business known as You Win USA Vacation Resort marketed to pregnant women who were then charged $50,000 for lodging, food and transportation. Investigators said women were also told to wear loose clothing to hide their pregnancies, and they were promised Social Security numbers and U.S. passports for their babies before returning to China. In one instance, a trainer in China helped fabricate employment and income information for an undercover federal agent posing as a pregnant client to secure a tourist visa. The undercover agent was encouraged to fly through Hawaii, where customs officers were believed to be more lenient than in Los Angeles, according to a copy of an affidavit in support of a search warrant. The business netted its owners hundreds of thousands of dollars in the past two years and helped Chinese tourists deliver more than 400 babies at just one Orange County hospital, the court papers said. Investigators obtained warrants for the searches in Orange, Los Angeles and San Bernardino counties hoping to collect evidence of suspected crimes, including visa and tax fraud, immigration officials said. Chinese Embassy spokesman Zhu Haiquan said in response to the crackdown that compared with "4 million people traveling between our two countries every year, these cases are sporadic. The Chinese government always requests overseas Chinese citizens to abide by the laws." Authorities did not release details of their findings or say how many women they found. Whether the women will stay here to give birth will be handled on an individual basis; authorities say some may need to remain as material witnesses. The key draw for travelers is that the United States offers birthright citizenship. Maternity tourists believe citizenship will help their children secure a top-notch U.S. college education and provide a sort of insurance policy should economic conditions crumble in their home country -- especially since the tourists themselves can apply for a green card once their American child turns 21. U.S. Customs and Border Protection warns on its website that officers at airports and on the border will consider a pregnant woman's due date, travel plans and medical insurance to determine whether she can enter the country. In Irvine, neighbor Linda Trust said she saw small groups of pregnant Chinese women together at the complex, and people bringing in platters of food and cases of diapers. "I don't think it's right," she said, adding that she had never seen any of the babies.