The southern border represents a potential vulnerability that terrorists could use to gain access to the U.S., according to Senate Homeland Security Chairman Ron Johnson (R-WI).

“My concern would be those Islamic terrorist operatives moving through Africa into Central or South America and then coming up through our completely porous border,” Johnson said in an interview with CNN’s Wolf Blitzer Monday evening.

He noted that Europe is currently “more vulnerable” to Islamic terrorism due to it’s proximity to Syria and its large influx of migrants but stressed that America is also at risk.

“You could have someone with a completely clean record, who’s been self-radicalized in Europe, using the Visa Waiver Program as well,” he said. “I think it’s more likely though that you move those individuals through our porous southern border.”

“We’ve actually got a category at Homeland Security called SIAs, special interest aliens,” Johnson added. “These individuals are being picked up in Central America — from Yemen and Somalia and Syria and from Pakistan. That’s where I think our greatest danger is.”

According to Johnson, ISIS has been effective at expanding, and confronting the threat is “extremely difficult.”

“We’re looking for a needle in a haystack. The problem is there are more needles and haystacks growing,” he said. “But again, I’m more concerned about the fact that ISIS continues to exist. They are extremely effective at using social media, and they use that to radicalize as homegrown terrorists here. And we saw that obviously in San Bernardino.”

Johnson added that while he does not agree with GOP presidential frontrunner Donald Trump’s call for a temporary ban on Muslim entry to the U.S., he believes there need to be improvements to the vetting process.