If, by X, you mean "go thru airport security". Luckily our borders are an impermeable wall stronger than the cold war era berlin wall, so there's no alternative method for bad guy to enter the country.
This is the comedy of the whole security theater system. In the old days generals always prepared to refight the last war, in the fatherland security era its about the same concept. The next attack will not come from a properly documented airline passenger, it'll come from "rio grande swimmer number 50 million and one".
May as well just let them fly even if we know they're bad guys, if they're going to get here anyway, one way or another, at least we'll make a profit off the ticket and its easier to track their actions at an airport than some desert out west.
Even worse, they're better off letting all the bad guys thru to boost budgets, and stop only good guys because its safer/less confrontational and makes good theater. If I know that, they probably figured it out a long time earlier, and almost surely implemented it.
Its also a bad tactical move. Lets say he was in fact a bad guy. The obvious standing order for a bad guy would be if you get caught, yet are not drone attacked or sent to the concentration camp in cuba, then being busted should be the immediate signal to enter our completely porous and open borders via non-traditional routes, and "do your thing" since you're obviously of no further use undercover. The mere fact this dude did nothing for over a year using this logic proves his innocence. If he was a bad guy then he would have "sneaked" in, gotten on campus as quickly as possible while his intel is still current, and (done something tragic)