Why wouldn’t they; it’s wide open.
Check it out:

Social media chatter shows Islamic State militants are keenly aware of the porous U.S.-Mexico border, and are “expressing an increased interest” in crossing over to carry out a terrorist attack, according to a Texas law enforcement bulletin sent out this week.

“A review of ISIS social media messaging during the week ending August 26 shows that militants are expressing an increased interest in the notion that they could clandestinely infiltrate the southwest border of US, for terror attack,” warns the Texas Department of Public Safety “situational awareness” bulletin, obtained by FoxNews.com.

The three-page bulletin, entitled “ISIS Interest on the US Southwest Border” and dated Aug. 28 was released to law enforcement on Thursday.