Federal agents walk past the Carlyle Apartments, the location of a suspected "baby tourism operation, in Irvine California March 3, 2015.[Photo/Agencies]

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 US 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 permanent residency once their American child turns 21.

US 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.

The whole chain of Chinese maternity tourism industry has been existing underground in Southern California for several years, from recruiting pregnant women coming to the United States to help them go to hospital until they recover and return to China with their babies.

Irvine, Rowland Heights and Walnut all have big Chinese communities. Chinese food stores and Chinese language services are convenient in these cities.

"Such businesses provide travel and lodging services to pregnant foreign nationals interested in coming to the United States to give birth so their offspring will be American citizens, " said Kice, adding that Tuesday morning's operation was designed to gather "further evidence related to a variety of possible criminal violations, including visa and tax fraud, money laundering, and conspiracy."

Chinese pregnant women usually pay several to dozens of thousand dollars for the services of those "maternity tourism" companies to help them get US visa, get a place to live, go to hospital and other needs in the United States.

The federal agencies involved in Tuesday's actions included ICE Homeland Security Investigations, the Internal Revenue Service, US Customs and Border Protection, the Department of State's Diplomatic Security Service, the Los Angeles Airport Police Department, the Irvine Police Department and the San Bernardino County Sheriff's Department.