Sneaking Islamic terrorists into the United States isn't that difficult. It's not that expensive. A people smuggler in the Middle East or Pakistan will do it, or make an attempt, for about $5,000. But there's a major problem with using people smugglers; there's a big chance you will get caught. Now, normally, those Asian, Middle Eastern or African people trying to sneak into the United States, who get caught, just get sent back home. But many of the countries that might catch them, first check fingerprints against those of known terrorists. Even if an Islamic terrorist making the trip has never been fingerprinted, interrogators might still be able to uncover his real identity. Once one of these terrorists is caught and identified, they end up getting interrogated, and information is revealed. This can snowball into a rollup of many more terrorists.
Thus the people smuggling route is long and dangerous. The most popular one gets the illegals into South America by ship or commercial airline, as tourists. Countries from Colombia, north, are popular arrival sites. You can bribe the border security people, if they get suspicious about so many Afghan or Nigerian men coming in, alone, as tourists. The people smuggler then moves the "cargo" north, one country at a time, until the U.S. border is reached. That's one of the biggest hurdles, because a lot of the illegal border crossers there get caught because of increased vigilance. Islamic terrorists cannot afford to get caught, and for that reason, it is believed that very few Islamic terrorists have been sent on this route, although there is evidence that it has been discussed by terror groups. It is believed that Islamic terrorists have been sent to South America, but many countries down there are easy to get into, and the corruption (ability to bribe suspicious security officials) makes it possible for Islamic terrorists to remain, at least as long as they don't kill anyone.