Criminals love US real estate. It’s been well documented.

Thanks to a loophole in the PATRIOT act, which forces banks to do considerable due diligence on any client but was never extended to the real estate sector, any ne’er-do-well can set up a shell company to buy properties in cash without ever revealing their identity.

Iran exploited the flaw to use a $500 million Manhattan skyscraper as a slush-fund for 22 years. Potential money-launderers have used it to anonymously snap up $1.5 billion worth of apartments either owned or branded by the Trump Organization. And the criminals who stole billions from Malaysia’s state development bank employed it to buy swaths of properties in New York and Los Angeles.

The Treasury set up a pilot program to tackle this in 2016, with a rather modest measure: It forced title insurance companies to find out who actually owned LLCs buying luxury real estate in a handful of US cities, and to give that information to the Treasury.

Despite being unambitious in scope, the program seems to have had considerable success. The Treasury said last year it has provided “valuable data” on real estate purchases by people tied to “foreign corruption, organized crime, fraud, narcotics trafficking, and other violations.”

It also seems to have had an effect on the market. Since the scheme started, there has been an almost 70% drop in companies buying real estate with just cash, according to an academic paper published last year. Those figures suggest that “anonymity plays a significant role” in people using secretive shell companies to buy property, and money-laundering was a “likely cause” for wanting that secrecy, Ville Rantala, a finance professor at Miami Business School and co-author of the paper, told Quartz after the paper’s publication last summer.

Despite this success, the scheme still isn’t permanent. Anti-corruption activists breath a sigh of relief every six months when the Treasury renews it. What’s more, it only applies to 12 cities. The fact that the Treasury keeps expanding the scope—they began by monitoring just Manhattan and Miami—to include even the likes of Honolulu and Seattle, suggests either that shady real estate business is more widespread than previously thought, or that criminals seeking anonymity are going to increasingly far-flung places to escape the authorities’ watching eye.

The idea that the pilot regulations should be implemented nationwide and permanently is a “no-brainer,” said Zoe Reiter, Transparency International’s acting US representative. “We know that they genuinely deter laundering of dirty money in the real estate market without impacting clean investment. The cost-benefit ratio for Americans is only positive.”

In a statement, a Treasury spokesman said the orders have a statutory limit of 180 days before they have to be renewed. However, that doesn’t stop them from expanding the regulations across the United States. The Treasury could also tackle the issue by ending the “temporary” exemption from the PATRIOT Act’s strict regulations given to the real estate sector when the law was passed nearly 18 years ago.

This is more than just a moral issue—it has national security implications. Rantala pointed to Russians hit by sanctions after the Kremlin’s efforts to sway the 2016 presidential election. “If there are these kinds of loopholes, it’s hard to know whether people targeted by sanctions may be able to make purchases in the US—so there’s really cause to be concerned about anonymous buyers,” he said.