The restrictions were further tightened in June, when the number of acceptable identity documents was narrowed from thousands — including common driver's licenses and birth certificates — to just a handful. Documents accepted from citizens for re-entry are now generally limited to passports, passport cards and trusted-traveler documents, such as SENTRI passes, along with the radio-chip-enhanced driver's licenses issued by some states, though not California.

Authorities say it doesn't appear to be a case of those trying to sneak in not getting the message.

“I think the word has gotten around,” said Angelica De Cima, a spokeswoman for the agency, who said that even those who try to falsely state they are U.S. citizens are producing some sort of fraudulent documentation to back up the claim. “They are coming in with documents because they have heard that if you are a U.S. citizen, you have to have a document now.”

The fact that fewer documents are accepted is probably driving more arrests, agency spokeswoman Jacqueline Dizdul said, because customs officers now only have to focus on a few types of identification.

“I think it's clear that narrowing the number of acceptable documents … to a handful assists our officers as they work to weed out fraudulent documents from legitimate documents, leading to more apprehensions,” Dizdul said.

Most new documents accepted are also machine-readable, making it easier to weed out false ones, she said.

The new document requirements applied beginning June 1 to U.S. citizens returning through land or sea ports of entry from Mexico, Canada, Bermuda and the Caribbean. Birth and naturalization certificates are still acceptable for minors younger than 16, and the rule doesn't affect legal permanent residents. Tribal identification is acceptable, as is military identification provided that a service member is traveling under orders.