The knee jerk reaction is, "Fat chance!" However, according to Breitbart.com, it depends on which cartel. In How and Why ISIS Will Use Texas to Enter the US If They Choose to Cross the Border, Brandon Darby & Ildefonso Ortiz explain that the cartels farther west have older leaders who wouldn't sacrifice future profit for a chance of a short term payoff. Whereas the cartels operating southeast of El Paso are now run by younger guys looking for the fast buck. They would be the ones most likely to escort Islamic extremists across the Texas border at a vulnerable spot, say, between Del Rio and Laredo.
Here's how, according to the authors:
Border Patrol agents from both the Del Rio and the Laredo sectors told Breitbart Texas that they simply do not have the resources to secure the area. They said that their resources are stretched so thin that only large groups are followed and tracked. The agents see “sign,” footprints or other indications that an illegal crossing has occurred, an unless the sign indicates a large group, the agents often do not have the ability to track and apprehend the smugglers or illegal aliens. This area is another region where an ISIS member or small group could pay a substantial sum to Zetas smugglers to secretly enter the U.S. without ever being known to authorities. The Zetas cartel would send one smuggler with a large group of illegal aliens in one direction, and then another smuggler with one or two ISIS members in another direction. The Border Patrol would pursue the larger group and be forced to ignore the two sets of footprints created by the ISIS group. It is important to note that the State of Texas has done little to increase State Trooper or National Guard presence in this area. That state effort largely focused on the Rio Grande Valley Sector and is only now beginning to be seen in the southeastern side of the Laredo Sector.
They would then have to cover a lot of barren land by foot, motorbike, or all terrain vehicle, so our best hope is that they would be too lazy to do that.