AMID all of the xenophobia and nativism surrounding the immigration debate, there is a real security concern. In the language of the bureaucracy, the problem is referred to as the “O.T.M.’s,” or Other Than Mexicans.

Thousands of non-Mexicans are caught crossing the United States border every year. They cannot be sent back to Mexico, but must be deported to their home country. Until recently, most were given a deportation hearing date and then simply released. Not surprisingly, few showed up for their scheduled appearances. Beginning last year, however, most who are caught are put into detention. They are then put through a procedure called expedited removal, under which many are flown back home within a few weeks.

Many of these non-Mexicans come from Iraq, Pakistan, Yemen, the Palestinian territories and other areas of concern to counterterrorism officials. What we don’t know is how many others are evading the Border Patrol every year and what happens to them when they leave the border area. It’s not too hard to imagine that these illegal immigrants, who have clearly spent a lot of money getting to Mexico and then into the United States, are able to buy themselves an identity and corroborating papers once in an American city.

Since 9/11, it has been far more difficult to get a visa to enter the United States if you are a citizen of a country considered a terrorism concern. But it is not difficult for a Pakistani, for example, to enter Mexico or another Central American country from which he can get to our border relatively easily, cross it and blend in.