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A group of 200 veterans and retired law enforcement officers have been patrolling the U.S.-Mexico border because they're concerned a terrorist could slip through.

Tim Foley, Field Operations Director of Arizona Border Recon, says it's not a matter of if, but when.

"We try to keep an eye on about 600 square miles of the Arizona border," Foley said on "Fox & Friends."

He responded to a sound bite from Jamiel Shaw Sr. whose son was killed by an illegal alien in 2008 and who penned a recent op-ed saying his son's murderer should have never been in the U.S.

"We need to make sure our citizens are protected before we allow all the protections for the illegals," Foley agreed.

He said that "everybody and their brother is coming through that border," noting that people from Afghanistan, Pakistan, Sudan and Somalia have come across.

But Foley pushed back against the idea that his group is anti-immigrant or "extremist."

"We're not against immigration, we just want it done the right way, because your friends wouldn't climb through the back window to come visit you. If you come through the front door that's fine, but we don't know who's who."

Watch the full interview above.

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