In the months following the implementation of new travel-document requirements at U.S. land and sea port of entries last June, there was a spike in the number of people arrested along the southern border posing as U.S. citizens, customs officials say.

U.S. Customs and Border Protection Officer Nelson Perez checks documents for several passengers in this northbound van at the San Ysidro port of entry.Oral declarations of U.S. citizenship are no longer accepted. (Photo: John Gibbons / Union-Tribune)

Between June 1 and the end of August, the latest period for which information is available, there was a 30 percent increase compared with the same period a year earlier in the number of people who tried to enter illegally by either declaring themselves to be U.S. citizens, posing as citizens using someone else's documents, or using phony ones.

U.S. Customs and Border Protection officials aren't sure why the increase has occurred, but believe it is tied to the introduction of stricter document requirements at the border that began last year. Among other things, it has been a year since oral declarations of citizenship, once an accepted practice, were ruled out and travelers were required to present some sort of identification.

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.

The change is part of a policy called the Western Hemisphere Travel Initiative, an outgrowth of national security legislation enacted more than five years ago. Passports have been required for air travelers returning from within the designated region since January 2007.

Oral declarations of citizenship have not been acceptable for re-entry by land or sea since last January.