America faces a grave risk from terrorists exploiting the nation’s vulnerable southwest border with Mexico. That’s the judgment — not of some conspiracy-minded wacko — but of Jeh Johnson, President Barack Obama’s former homeland security chief.

Johnson laid out his concerns in a previously unreleased June 2016 memo written during his final months as secretary of the Department of Homeland Security. The Center for Immigration Studies (CIS) included the memo in a report Monday highlighting the link between immigrant-smuggling and terrorism threats.

Titled “Cross-Border Movement of Special Interest Aliens,” the three-page memo to a host of immigration and national security officials called for “immediate attention” to “counter the threats posed by the smuggling of SIAs.”

Johnson called for a “Consolidated Action Plan” because “as we all appreciate, SIAs may consist of those who are potential national security threats to our homeland. Thus, the need for continued vigilance in this particular area.”

Todd Bensman, a senior national security fellow at CIS, told LifeZette that Johnson’s memo suggests there is good reason to be concerned about the potential of terrorists to sneak across the U.S.-Mexican border.

“The Obama administration was the one that saw this particular traffic coming across the border in its last year,” Bensman said, referring to notice given to individuals with potential terrorist links who are described as “Special Interest Aliens” (SIA).

The federal government’s SIA definition has changed over time and usually has been based on countries designated as those that pose a high level of national security concerns.

Quantifying the potential problem is difficult because of a lack of public reporting. But Bensman in his study cites several government reports that offer clues.

One set of SIA apprehension data suggested that U.S. officials detained nearly 6,000 people from 42 high-risk countries from the time of the Sept. 11, 2001, attacks through 2007.

A 2009 Government Accountability Office (GAO) report identified 530 SIAs had been logged at border checkpoints the previous year, including three “identified as linked to terrorism,” according to the CIS report.

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Bensman, who formerly managed teams of intelligence analysts for the Texas Department of Public Safety’s Intelligence and Counterterrorism Division, said he has not spoken to Johnson.

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But he said senior U.S. government officials very likely watched with alarm as terrorists insinuated themselves into the massive flows of refugees that streamed into Europe from the Middle East during the Syrian civil war.

Some of those terrorists have carried out attacks in Europe.

“That’s sort of informed or educated speculation … It’s not farfetched to think they would want to replicate that here,” he said.

It is unclear what, if anything, President Donald Trump’s administration did with the process that Johnson began. Independent national security experts said the government would be wise to focus on the problem.

“Speaking in general, it’s absolutely an issue of concern,” said Kyle Shideler, director of the Middle East Forum’s Counter-Islamist Grid (CIG).

Shideler dismissed the notion that the threat has been exaggerated, given the lack of terrorist attacks carried out by illegal immigrants smuggled across the border.

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“Historically we have seen cases where known or suspected terrorists have crossed the southern border illegally or made plans to do so, including terrorist operatives with ties to groups like Hezbollah, al Qaida and al-Shabab,” he said. “That’s a threat that has to be taken seriously.”

Bensman said there are a variety of reasons why the smuggling operations have not resulted in terrorist attacks. He pointed to documented cases where law enforcement authorities successfully have intervened before plots turned violent. He said U.S. Border Patrol officers likely have caught others shortly after entering the United States.

“But what’s indisputable is the capacity is there,” he said.

Bensman said the issue deserves the attention of multiple agencies across government, not just officials tasked with immigration enforcement. “That’s their job,” he said. “We pay them to be experts and think about this sort of thing.”