From potential terrorists who enroll at phony schools only to melt into the U.S. population, to foreign scientists who come to study weapons technology at America’s top schools, the student visa program is allowing dangerous enemies into the country, a former top federal official told FoxNews.com.

Recent attention has been focused on refugee programs and illegal border crossings, but the Achilles heel in America’s immigration system may be the program that invites 1.2 million foreigners into the U.S. each year, according to Claude Arnold, retired special agent in charge for Immigration and Customs Enforcement’s Los Angeles bureau of Homeland Security Investigations. Once here on student visas, immigrants are barely monitored and tens of thousands don’t show up for classes and fall off the government radar.

“Our legal immigration system has many vulnerabilities and the student visa program is no different,” Arnold said. “It is only a matter of time before there is either some horrible criminal act, or some act of terrorism, and there is absolutely no information available that would have caused [authorities] to go out and pick that person up.”

“It is only a matter of time before there is either some horrible criminal act, or some act of terrorism, and there is absolutely no information available that would have caused [authorities] to go out and pick that person up.” — Claude Arnold, retired ICE special agent in charge

Most of student visa recipients do exactly what they said they would do when they applied – take advantage of America’s vaunted system of higher education and leave when the terms expire. But every year, approximately 58,000 overstay their visas and drop out of contact with authorities. While the vast majority of those are not terrorists or spies, some are, said Arnold.

ICE’s 7,000 agents simply don’t have the ability to monitor all of them, Arnold said. By the time a red flag goes up, it may be too late.

“You have to conduct a threat assessment and go after those who are a threat to national security,” Arnold said. “But within that universe of people who are visa overstays, there could be people who are radicalized, and we just don’t know it because there is no intelligence on them,” he added.

Foreign enemies know how to exploit the student visa program, Arnold said. Iran, in particular, has sent scientists to the U.S., ostensibly to study other subjects, but really to gain knowledge to benefit Iran’s weapons program.

“My concern was we had Iranian students who studied at Iran’s big physics school and were essentially nuclear physicists working on their bomb project,” Arnold said. “We had cases where they would register for a mechanical engineering class in the U.S., but really all they were trying to do is get access to an aeronautical engineering program, so they could work on the delivery system for Iran’s nuclear program.”

The State Department and the Department of Homeland Security share responsibility for screening applicants and monitoring them once they arrive.

ICE officials told FoxNews.com each school that takes in visa recipients has a designated official who serves as a point of contact between students, the school and the government’s Student and Exchange Visitor Program to ensure the federal computer tracking system is updated. In addition, 58 field representatives visit approved schools twice a year to ensure compliance.

The State Department, which oversees part of the student visa program, told FoxNews.com in an emailed statement it is committed to a “transparent and efficient visa application process,” and maintains extensive programs to vigorously combat and investigate visa fraud.

Fraud prevention managers engage in public outreach, training, detailed review of cases, statistical analysis and other activities, including communicating with host government officials and U.S. law enforcement authorities, the statement said.

Applicants are screened by a host of federal agency databases and personnel against databases of fingerprints of known and suspected terrorists, wanted persons, immigration law violators, and more than 75.5 million criminal history records.

ICE statistics show countries sending their students include several considered by the U.S. as State Sponsors of Terrorism, including Syria and Iran, as well as Saudi Arabia, China and Pakistan. More than 700 Syrians came to the U.S. via the student visa program in 2014, and another 3,700 came from Iran the same year.

“We don’t really know if State’s efforts are effective or if they are helping reduce fraud and abuse of visa programs, because Department of Homeland Security refuses to release a report detailing the number of overstays in each visa category and from each country, even though Congress has mandated this report since at least 2004,” said Jessica Vaughan, a former State Department consular officer who now is the director of Policy Studies for the Center for Immigration Studies, a Washington, DC-based research institute.

“What I am worried about is students who are allowed to get a student visa to attend some nondescript school and then they disappear,” said Vaughan, who noted that Hani Hanjour, the 9/11 hijacker who flew Flight 77 into the Pentagon, had obtained a student visa but never showed up for class.

Too often, schools play along. ICE has cracked down in recent years on “visa mills,” or facilities that help foreigners get a student visa for a fee, but never hold classes or ensure students attend class. Three California residents pleaded guilty last March in a "pay-to-stay" scheme involving three sham schools in Los Angeles.

The schools had legitimate-sounding names, like Walter Jay M.D. Institute and the American College of Forensic Studies, and took in millions of dollars in tuition fees. But investigators found classes that were supposed to hold 30 students had just few, if any students. According to Arnold, who oversaw the investigation, the schools existed only to facilitate foreign students’ purchase of visas under the guise of studying.

“This is an example where the system worked,” said Arnold.

In addition to better screening and monitoring, Arnold believes overstaying a visa should be a misdemeanor. That might make visa holders less likely to violate the terms, and would also trigger alarms if they were stopped for a traffic violation or arrested for another reason.

Arnold and Vaughan also want tighter controls on the kinds of schools that can accept foreign students. Some trade school programs that teach subjects like massage, baking and horseshoeing could invite fraud. And courses that teach material with military applications invite something even more sinister, Arnold said.

“Why do we want people who are our enemies, whether it is potentially ISIS or Iran, here learning technical skills they are going to use against us?” he said. “It is insane.”