Only a matter of time.
Very likely that they’ve already crossed into the country.

RETIRED LT COL: ‘TERRORISTS WILL MOST CERTAINLY’ CROSS THE BORDER
Ret. Lt. Col. Tony Shaffer said that the easiest way for terrorists is “absolutely” the easiest way to get across the southern border, and that that “terrorists will most certainly” cross the border on Monday’s “Your World with Neil Cavuto” on the Fox News Channel. “Drug dealers are already doing it. It’s just a matter of time [before terrorists enter through the border] and, Stuart, let’s be totally real here, they [drug cartels] have already done partnerships with Hezbollah to get operatives in.”
Shaffer did point out that “there’s no known case of where a cell has popped up and gone operational,” but cautioned, “We need to understand that if drug groups can do it [cross the border], if gangs can do it, terrorists will most certainly be doing it, and so the president has to understand that the threat is already real.”
More here.