A report by the Texas Department of Public Safety raises concerns about ISIS terrorists using the Mexican border both to enter and leave the country. It noted that at least 13 aspiring terrorists have tried to cross into Mexico, or considered trying, since 2012.

Most of those cases involved people who knew they were on the federal no-fly list but wanted to travel to join terrorists, the report said. Sneaking across the southern border "presents an opportunity for increasing numbers of aspiring foreign terrorist fighters to evade US interdiction efforts such as the No-Fly List."

The most recent example happened in October. Texas authorities arrested two Milwaukee men near San Angelo, Texas on the way to the Mexican border. Jason Ludke, 35, and Yosvany Padilla-Conde, 30, wanted to go to Mexico, obtain fraudulent travel documents and travel to join ISIS in Syria or Iraq.

In another instance in April 2015, seven Somali men from Minnesota tried to cross from San Diego into Mexico in an effort to get to Syria and fight for ISIS.

Texas resident Bilal Hamed Abood, an Iraq-born naturalized U.S. citizen, successfully used the border in 2013 to travel to Syria, where he fought for a Syrian rebel group. The FBI arrested Abood for lying about his initial travel to Syria when he tried to come home through the Dallas-Fort Worth airport. Abood claimed he fought for a faction that was not prohibited under U.S. law. However, FBI agents search his computer and found that he took an oath of allegiance to ISIS leader Abu Bakr al-Baghdadi.

Texas authorities voiced concern in 2014 about ISIS social media threats to use the Mexican border to enter the United States.

In a criminal complaint filed last year, alleged ISIS supporter Erick Jamal Hendricks claimed to have had contact with an ISIS supporter known as "Abu Harb." "Abu Harb" told Hendricks that he was in Dallas and that the "Islamic State had brothers in Mexico."

Previously, government officials warned about threats to the U.S. border posed by other terrorist groups including Al-Shabaab and Hizballah.

President Trump touted the ISIS threat as a reason for building his wall along the Mexican border during the campaign. He signed an executive order Wednesday calling for the wall's construction, but funding sources are not yet clear.