LOS ANGELES—Federal agents Tuesday executed search warrants at several Southern California sites they say are connected to three multimillion-dollar birth-tourism businesses that enabled thousands of Chinese women to travel here and return home with infants born as U.S. citizens.

The investigations are likely to culminate in the biggest federal criminal case ever against the booming “anchor baby” industry, according to U.S. authorities. The search warrants cite suspected visa fraud, tax evasion and harboring illegal immigrants, among other charges.

Agents seized records from apartments where, they said, Chinese women on tourist visas stay before and after delivering babies, as well as from residences of U.S.-based individuals who they allege run three separate anchor-baby operations in Los Angeles, Orange and San Bernardino counties. The U.S. Homeland Security and Treasury departments, as well as the Internal Revenue Service, are conducting criminal investigations of these individuals, according to three affidavits reviewed by The Wall Street Journal.

In Orange County on Tuesday morning, about 40 agents entered the luxurious Carlyle Apartment complex, where residents said they had noticed many pregnant women coming and going. The agents, who collected diaper boxes, computers and documents, are expected to remain at the site all day. No arrests were observed.

The businesses named in the affidavits could not be immediately reached for comment. One of the companies, Los Angeles-based Star Baby Care Center says on its starbabycare.com website that it has “served” more than 8,000 women since 1999. Another company, USA Happy Baby Inc., promises to refund women if U.S. officials at the airport send them back home, according to the affidavit.