IRVINE, Calif. (AP) — Federal agents searched three dozen homes Tuesday in California during a crackdown on so-called maternity tourism operators who arrange for pregnant Chinese women to give birth in the U.S., where their babies automatically become American citizens.

The investigation of three alleged birth tourism rings may be the biggest yet by federal homeland security agents who say that, while pregnant women may travel to the United States and deliver their babies here, they cannot lie about the purpose of their trip when applying for a visa.

Authorities believe people from other countries are carrying out similar schemes but recent cases in California have catered to wealthy visitors from China, most likely due to the country's large population, recent economic boom and ties to the region. It is unclear how many women travel to the United States for maternity tourism.

"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 coached to lie about their travel plans when applying for tourist visas and 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 her 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 American babies at just one Orange County hospital, the court papers said.

No arrests were made or charges filed. 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. They did not release details of their findings.

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.

Dr. Jin-Jou Lu, who also lives in the complex, said he wasn't surprised to learn of the scheme.

"Come on, people go across the border to have a baby from Mexico all the time, so what's the problem?" he told reporters.

Federal agents started investigating the business in Irvine after an anonymous tip last year. During the investigation, they tracked the movements of a couple who arrived in February 2014, had their baby in April and returned in May. While the couple's bank account recorded charges at luxury stores including Louis Vuitton and Rolex, they paid $4,080 — less than 15 percent of the billed amount — to an Orange County hospital for medical services after stating the mother was not employed, the affidavit said.

In 2013, Los Angeles County cited more than a dozen maternity hotels for code violations after an uproar in a nearby suburban community about a hotel operating in a residential neighborhood.