By now, it’s common knowledge the National Security Agency collects plenty of data on suspected terrorists as well as ordinary citizens. But the agency also has algorithms in place to filter out information that doesn’t need to be collected or stored for further analysis, such as spam emails—a fact terrorists used to their advantage.

Much of the debate around the NSA’s overreach has focused on selectors, the terms it uses to describe its requests for information collected. According to a transparency report it published last summer, the agency was approved to use 423 selectors in 2013 under its telephone metadata program. However, filters, which specify data the agency does not want, also play an important role in reducing noise.

In a paper published by the American Mathematical Society, the agency’s former research director, Michael Wertheimer, recalled an instance when the US seized laptops left by Taliban members soon after the 9/11 attacks. The only email written in English found on the computers contained a purposely spammy subject line: “CONSOLIDATE YOUR DEBT.” According to Wertheimer, the email was sent to and from nondescript addresses that were later confirmed to belong to combatants.

“It is surely the case that the sender and receiver attempted to avoid allied collection of this operational message by triggering presumed ‘spam’ filters,” he said, noting the agency is constantly refining its algorithms to discover new threats.