One of the suspects charged in connection with a Muslim extremist compound in New Mexico highlights a huge vulnerability that national security and immigration experts have been harping on at least since the 9/11 attacks.

Jany Leveille (pictured center above) came to the United States from her native Haiti two decades ago on a visitor visa and then stayed long after it expired, according to Immigration and Customs Enforcement (ICE) officials.

Leveille exploited the same weak visa enforcement system that enabled the 9/11 hijackers to be in the U.S. without detection, and the 911 commission recommended closing loopholes associated with nonimmigrant visas.

Yet, the U.S. still has not implemented a functioning system to track new visitors, much less find and deport people who came years ago and remained here long after their legal visitation ended.

“This case is not isolated,” said Michael Cutler, a former officer for the old Immigration and Naturalization Service (INS).

Cutler, who testified before the 9/11 commission, told LifeZette he believes vested interests have blocked efforts to improve tracking of visitors. He said he believes business interests fear that greater scrutiny would lead to calls to scale back or eliminate the visa waiver program, which exempt citizens of certain countries from having to obtain visas to travel to the United States.

Related: Woman Arrested at New Mexico Jihadi Compound Was in U.S. Illegally (Stunning Details)

“The reason that was never done, in my judgment, is the [fears over losing the] visa waiver program … We don’t even know how many illegal aliens are in the United States,” he said. “That 11 million figure is a lot of nonsense.”

Law enforcement authorities arrested Leveille, 35, and four other people on August 4 after raiding the compound in rural Amalia, New Mexico. Investigators found two men, three women and 11 children living in squalid conditions.

Authorities have alleged that the defendants were training the children for one or more mass shootings. The defendants may have had in mind imitating the Beslan Massacre in Russia in 2004 in which more than 1,100 people, including 777 school children, were held hostage. At least 334 were killed during the three-day attack by Chechen Islamic terrorists.

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Leveille moved to Brooklyn, New York, in 1998 after her father died, according to an account provided to Reuters by her brother. She married, separated from her husband, and then lived at times in Georgia, Philadelphia, and New York.

Prosecutors said that among those at the compound with her was her current husband, Siraj Ibn Wahhaj, and their children.

Matthew O’Brien, director of research from the Federation for American Immigration Reform (FAIR) said the fact that Leveille even was able to marry owes to an inconsistency in the law.

“We don’t even know how many illegal aliens are in the United States. That 11 million figure is a lot of nonsense.”

Had she entered the United States without permission, she could not have used her marriage to an American to adjust her status to legal residency. And people who overstay visas generally are not allowed to adjust their status for other reasons, such as a job offer.

But people with expired visas can obtain green cards by marrying Americans, O’Brien said.

The revelations about Leveille come a week after the government reported that 700,000 foreigners had overstayed visas that expired in fiscal year 2017. As of May 1 of this year, an estimated 421,325 were still in the United States.

Related: More Than 700,000 Foreigners Overstayed Their U.S. Visas in 2017

“Visa overstays, we know, cumulatively are a big category of illegal aliens,” said Clare Lopez, vice president of research and analysis at the Washington-based Center for Security Policy. “It’s a hard problem to stay on top of.”

Lopez said it points up the need not just to do a better job of making sure visitors honor their visas on the back end but also making sure U.S. officials are applying sufficient scrutiny on the front end.

“It’s tough, but I think maybe the questions need to be more stringent,” she said.

Despite congressional mandates, the government has not implemented a biometric entry/exit system to track visa overstays.

Even if such a system were in place, it is unclear whether ICE agents would have knocked on Leveille’s door once her visa expired.

“This woman probably would not have been a high priority based on the way we’ve done things,” said O’Brien, a former assistant chief counsel with ICE.

But O’Brien said aggressive enforcement of visa rules would spur more voluntary compliance. He suggested that Leveille’s background should change the way Americans think about potential terrorism threats.

Related: Judge in New Mexico Compound Case Says Prosecutors Are Anti-Muslim (and It Gets Even Worse)

“It shows pretty clearly that it’s a huge threat, and it’s a huge threat that’s not necessarily coming from the Middle East,” he said.

Todd Bensman, a national security fellow at the Center for Immigration Studies (CIS), said it is common for people who have stayed beyond the expiration of their visas to float below the radar.

“A lot of that has to do with whether someone has come to the attention of law enforcement,” he said.

If not, Bensman said, foreigners with expired visas easily can “disappear into the ether.” When they finally do come to the attention of authorities, he added, the chances are higher that they will have families and community ties that form the basis of an argument that they should be allowed to stay.

“There’s a reward for evading the rule of law long enough to be able to establish roots,” he said.