IRVINE, Calif. (AP) — Federal agents searched more than a dozen homes Tuesday in 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 crackdown on three alleged maternity tourism rings may be the biggest yet by federal homeland security agents who say that, while pregnant women may travel to the United States, they cannot lie about the purpose of their trip when applying for a visa.

Birth tourism has been reported from a range of countries, but authorities say the most recent cases in California have catered to wealthy Chinese amid a boom in tourism from mainland China. It is unclear how many women travel to the United States for maternity tourism.

Shortly after sunrise Tuesday, federal agents swarmed an upscale apartment complex in the Orange County city of Irvine where authorities say a birth tourism business charged pregnant women $50,000 for lodging, food and transportation.

Investigators said women were coached to lie about their travel plans when applying for tourist visas and were promised they would receive 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 and 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 Orange County hospitals, the court papers said.

No arrests were made or charges filed. Investigators conducted the searches in Orange, Los Angeles and San Bernardino counties, hoping to collect evidence of crimes tied to three separate maternity tourism operations, including visa and tax fraud, U.S. Immigration and Customs Enforcement said.

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.

Bill Ong Hing, professor of the University of San Francisco School of Law, said wealthy foreigners from across the world often travel to the United States for medical care and are allowed to do so on visitor visas, provided they don’t overstay.

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 the Irvine case, investigators said another undercover officer posing as a pregnant traveler was told not to apply for her visa too late as she would be denied if she were noticeably pregnant. A pregnant woman who was questioned at the airport said she was told to say she was simply a tourist, court papers said.

Investigators also 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 the hospital for medical services after stating the mother was not employed, the affidavit said.

Federal agents started investigating the business after an anonymous tip last year.

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.

Jon Feere, legal policy analyst for the Center for Immigration Studies, said he believes birth tourism will grow along with international travel.

“There is a sense the foreigner has control over our immigration policy rather than the American people,” said Feere, who wants stricter limits on immigration.