An exterior view of Carlyle Apartments, a location suspected of involvement in "maternity tourism" schemes, in Irvine March 3, 2015. [Photo/Agencies]

"If someone is designated a material witness, their continued presence will be required," she said. "We encountered numerous prospective witnesses at all these locations, not just clients."

She said Mandarin interpreters were assisting in the probe.

The sweep was believed to mark the first such enforcement against an illicit cottage industry that has gained a growing foothold in the United States in recent years while operating largely out of sight of federal authorities.

The US Constitution grants citizenship to any child born on US soil, regardless of parentage, and immigration experts say there is nothing inherently illegal about women from abroad coming to the United States to give birth.

But operators of the maternity hotels are suspected of obtaining non-immigrant US visitor visas under false pretences, as well as failing to report income derived from their services to tax and banking authorities.

Tuesday's sweep targeted operators catering mainly to wealthy Chinese women who paid $10,000 to $80,000 for travel, lodging, medical care and documentation services advertised on the Internet, officials said.

One such service known as Baby Star Care advertised itself has having served 8,000 pregnant women, including 4,000 from China, since 1999, according to one government affidavit.