The volume of immigrants caught illegally crossing the U.S.-Mexico border is at a historical low, reflecting statistics not seen since the 1970s.

Except in South Texas.

The Rio Grande Valley, the deepest corner of Texas abutting Mexico, actually saw an increase of undocumented immigrants last year — nearly 100,000 arrests, a significant increase from close to 60,000 in 2011.

And in the Border Patrol’s Del Rio sector, also in South Texas, arrests also jumped in the same period, though more moderately.

All other areas of the U.S.-Mexico border saw big drops in immigrant arrests. So what’s up in Texas?

The Border Patrol’s answer: As elsewhere along the border, the count of Mexican crossers is down, but in Texas the flow of “OTMs,” or “Other Than Mexicans” — mostly Central Americans — is actually up. Way up.

That is a new trend that we are looking at closely,” said Daniel Tirado, spokesman for the agency’s Rio Grande Valley sector. “We had more OTM’s than Mexican nationals.”

Of the nearly 100,000 arrests in the sector, nearly half were marked as OTMs — an unusually high percentage. In the border’s busiest sector, Tucson, Ariz., less than 18,000 of the total 120,000 arrests last year were OTMs.

While nationwide Border Patrol has seen a 50 percent decrease in arrests compared to 2008, the upswing in Texas can’t be attributed to a single factor, agency spokesman Tirado said. It’s more likely a combination of various issues, including the increasing count of Central American youngsters escaping violence at home.

In the past three to four years, the number of immigrants that travelled through the Arizona and California border had boomed. This increase in numbers also resulted in a higher number of immigrants who died along the way, said Tom Power legal advocate for the Texas Civil Rights Project.

One of the main factors that can be tied to the shift in routes is an increase in border security in other areas such as Arizona, forcing more immigrants to South Texas, Power said.

The increase in immigrants from Central and South America can be tied not only to the poor economic situation there, but also to spillover violence from Mexico caused by drug cartels expanding their territory.

“Guatemala, El Salvador and Honduras have been subjected to an increase in crude violence as the Zetas and the Sinaloa cartel fight for control of those areas,” said Correa-Cabrera said.

For some OTMs, particularly those trekking from South America, it’s simply about getting to the nearest border-crossing spot — and traveling north through Mexico, the easiest area to reach is the Rio Grande Valley in South Texas.

Traveling to crossing points in New Mexico, Arizona or California would imply more time and money.

Ildefonso Ortiz is a freelance writer based in McAllen, Texas.

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