The surge of migrants across the southern border — including those who aren't caught by immigration officials — presents a clear national security threat, California officials told Fox News.

"I don't think it's out of the question to believe that a Hamas-style attack that happened Oct. 7 could happen to us," El Cajon Mayor and congressional candidate Bill Wells said. "That another 9/11-style attack or something we've never even seen before could happen."

"I'm not trying to scaremonger, but I think you'd have to be foolish not to believe that," he added.

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Border Patrol Chief Jason Owens last month described not knowing who is crossing the southern border illegally as a "national security threat."

"What’s keeping me up at night is the 140,000 known gotaways" so far in fiscal year 2024, Owens said during a "Face the Nation" interview on CBS, referring to migrants who trigger sensors or are seen on camera crossing the border, but are not taken into custody.

Border Patrol doesn't know what those individuals are bringing into the country, nor what their intentions are in the United States, he said.

"Those things for us are what represent the threat to our communities," he said. "Border security is a big piece of national security."

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Authorities arrested Fredy Ivan Jandres-Parada, described by the FBI as a senior leader of MS-13, last month at the California-Mexico border after more than three years on the run, the Los Angeles Times reported. Two weeks later, Border Patrol agents reported catching another MS-13 member trying to sneak into the country near Calexico, just east of the agency's San Diego sector.

"I'm glad they caught this individual," San Diego County Supervisor Jim Desmond said of Jandres-Parada. "But how many others have come across that we don't know of?"

Desmond has visited the border and transit stations where migrants have been dropped, seeing and talking with people from Haiti, India, Pakistan, China and other countries. 

"Communist China is not always our friend, and so there's tensions there," he said. "To have many of their people just walking across our border — maybe they have good intentions, but maybe they don't."

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Desmond also argued "proper vetting is not taking place," pointing out that border officials can only check crime and terrorism databases from the United States and some of its allies.

"They don't have China's database. They don't have Pakistan's database or Venezuela, that database, to check to see if these people have any type of criminal background," he said.

Nationwide, there were more than 3.2 million migrant encounters in fiscal year 2023, primarily at the southern land border, according to Customs and Border Protection data. Although monthly encounters have slowed since reaching a high of 370,899 in December, 2024 is still on track to set a new yearly record.

"Someday we're going to get a rude awakening," Desmond said. "Something's going to happen that involves people coming across the border that mean us harm and then being able to pull something off."

After the Sept. 11 terrorist attacks, Wells said "everybody was very security-minded."

"We never would have thought that we would allow people to just flood into the country with no checking on their background whatsoever and just say, 'No, it's all going to be fine because they need asylum,'" he said.

"We've changed so much in that short period of time, and I think we're going to pay a price for it," Wells added.

To hear more from Wells and Desmond, click here.