The recent ISIS terrorist attacks in Barcelona and Cambrils likely cost no more than a few thousand dollars—little more than the cost of renting vehicles and housing and feeding operatives for a few weeks. These attacks fit a pattern of high-impact, low-cost terrorism across Europe. Even the Paris attacks in November 2015, which killed 130 revelers, likely cost no more than $10,000.

It’s not clear whether the terrorists in Spain received money from abroad or raised funds locally to carry out the attacks. But we know that even as coalition military forces have scored victories against ISIS in Iraq and Syria, ISIS operatives and terrorists have turned to these kinds of low-cost attacks across the West. Just since the beginning of January, we have seen horrific examples in London, Paris, Manchester and Stockholm, in addition to the attacks in Spain.

These small-dollar attacks pose a new challenge for governments, which have developed effective strategies to target ISIS revenue sources since 2014. But they have less experience disrupting the financing of ISIS attacks that cost just a few thousand dollars. Given the small amount of money involved, stopping that financing requires a different strategy than the U.S. and our allies have previously developed. It’s not an easy task, but there are ways to stop small dollar flows and to block small-scale ISIS crime, smuggling and local fundraising. As ISIS loses territory in Iraq and Syria, it will redouble its efforts to retaliate with terrorist attacks in the West—and stopping the financing of these attacks is critical to our national security.

Between 2014 and 2016, the international community’s financial war against ISIS focused on the vast sums of money ISIS extracted from its control of territory in Syria and Iraq. U.S. officials estimated that, at its peak in 2015, ISIS was earning $50 million a month or more by selling oil in Iraq and Syria. ISIS took in hundreds of millions more by looting bank vaults, skimming off the top of Iraqi government payments to Iraqi civil servants living in ISIS territory, and extorting funds from businesses and individuals in the cities it controlled. By the end of 2015, ISIS was widely considered the richest terrorist group in history.

As the international military campaign against ISIS ramped up, however, the U.S. and its allies made remarkable progress in the fight against those sources of revenue. U.S. and coalition forces have bombed several thousand ISIS oil targets and other ISIS economic targets, such an an ISIS cash house that held millions of dollars of currency. Iraqi and allied forces have liberated towns and oilfields in Iraq and Syria, causing ISIS’ revenues from extortion to plummet. A study released this past February by London’s International Center for the Study of Radicalization and Political Violence, for example, estimated that ISIS’ revenues fell from nearly $2 billion in 2014 to less than $900 million last year. The recent liberation of Mosul and other ISIS-controlled cities has almost certainly pushed ISIS’ revenue even lower.

As a result, ISIS is seeking other sources of revenue and new ways to provide financial support to fighters. Recent U.S. and international criminal cases help to shine a light on the trends. For example, earlier this month the FBI shut down a scheme in which ISIS tried to use fraudulent sales on eBay to move money to a U.S.-based operative. U.S. and international authorities have stepped up efforts to stop ISIS’ smuggling of Syrian antiquities to black markets in the West. And ISIS is also working to raise money locally in the West. Over the past two years, European authorities have arrested a number of ISIS operatives suspected of raising small-dollar funds to help send European national fighters to join ISIS in Syria. As ISIS loses territory in Syria and Iraq and focuses on attacks in the West, this small-dollar fundraising will shift to support attacks in the West rather than supporting the flow of fighters to ISIS’ home territory.

This new ISIS finance threat involves far fewer dollars than the millions ISIS made in Syria and Iraq. And in many ways the smaller scale of this financial threat makes it harder for U.S. and Western governments to fight. The U.S. military has developed relatively straightforward protocols to identify oil wells and other economic assets in ISIS territory and to destroy them. Detecting the online transfer of a few hundred or a few thousand dollars, or stopping small-scale criminal money-laundering schemes, can be comparatively more difficult for law enforcement authorities. But as the string of ISIS attacks in Europe show, attacking ISIS’ new financial threat is critically important to national security.

Though stopping the transfer of just a few hundred dollars among the billions of small transactions taking place between individuals every day may sound impossible, there are actually new techniques for detecting them, leveraging growing computing power and big data to discover suspicious transactions. Banks today can use far more sophisticated algorithms than were available even a few years ago to spot second- and third-order financial connections between potentially suspicious individuals. They can also connect non-financial data, such as the IP addresses of computers that customers use to check their accounts, to look for suspicious patterns. For example, a bank may flag a European-national customer who checked his account from ISIS-controlled territory in Syria and then tried to send a small amount of money to an acquaintance back home. Banks are also getting better at using analytic techniques to identify signs of money laundering, such as intentionally overpaying for goods, and assessing whether such money laundering could be linked to terrorist activities.

But for all these improvements, there is still more the West can do to track and block small-dollar transactions that finance terrorist attacks. Some of these changes fall on banks, which can track money through the financial system, while others fall on the U.S. government and intelligence agencies. Through better coordination and more sophisticated use of data, the government and banks can make a real dent on this local fundraising, cutting off a critical component to ISIS of deadly attacks in the West. There are three big things we can do to work more effectively in the financial fight against ISIS.

First, financial institutions need more information about what kinds of transactions to look for—information the government, with its intelligence and expertise, often has to share. The Financial Action Task Force, one of the major international counterterror finance bodies, published a “typologies” report in early 2015 to give banks information about the red flags of ISIS finance but has not updated the report to address the major changes in ISIS funding over the past two and a half years. Both FATF and individual governments need to be regularly providing current information to the private sector.

Second, banks need to continue investing in analytic capacity that can help identify small-dollar transactions that may not initially appear suspicious but that fit patterns associated with ISIS fighters. While some large global banks have begun to upgrade their capacity in recent years, new analytic techniques and expanded computing power are not yet uniform across the financial industry, and the sector as a whole needs to make important investments.

Third, and perhaps most importantly, the U.S. government should follow the lead of the U.K., which has recently adopted new protocols to improve the sharing of financial information between government and the private sector. The U.K. recently established a Joint Money Laundering Intelligence Taskforce, which enables U.K. authorities to share classified information about money laundering and terrorism finance with banks and facilitates banks sharing information about suspicious patterns of transactions among themselves. Between mid-2016 and mid-2017, the task force contributed to 63 arrests and helped law enforcement identify more than 2000 previously unknown suspicious accounts. A similar organization in the U.S. would help American authorities and banks combat terrorism finance here and around the world.

Officials are not going to be able to stop every dollar that ISIS seeks to use to launch terrorist attacks against the U.S. and our allies. But the U.S. and our allies need to do all that we can to combat the most deadly terrorist threat we have faced since 9/11. Smart, effective steps to reduce the small dollars ISIS requires to mount its attacks can make them fewer in frequency and less likely to succeed.

Peter E. Harrell is an adjunct senior fellow at the Center for a New American Security.

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