The Casio F91-W digital watch pictured above, is cheap, with a list price of $18.95 and selling for $10.72 on Amazon as of this writing. It’s well-regarded, with well over 1,000 reviews on Amazon, nearly 90% of which are for four or five stars. It’s resilient, too: it is water resistant with a battery-life of seven years, and if you replace the battery at that point, according to many reviewers, you can get another seven years out of it easily. It’s even approved for military use and good for a practical joke at the same time, as one reviewer attested:

During Air Force Basic Military Training, nearly every trainee purchases this model watch from the BX. To Air Force dudes, the Casio F-91W is “THE BMT WATCH”. As a sign of solidarity, every member of my Fight perfectly synchronized all our watches and turned the “hour beep” on. During discussions with our MTI as a group, the room would be filled with the deafening perfectly timed BEEP on an hourly basis. It is hard to convey the sense of satisfaction this gave us. For that reason, the watch holds a special place in my heart. True story.

Everyone seems to like the Casio F91-W.

Even terrorists.

In 2011, the Guardian reviewed training manuals given to agents stationed at the U.S. military base on Guantanamo Bay, Cuba, where many suspected Al-Qaeda operatives are held. The documents, per the Guardian, “advise that possession of the F-91W [. . . ] suggests the wearer has been trained in bomb making by al-Qaida in Afghanistan.” Further, the Guardian explained, more than two dozen of the 150 or so people being held at Guantanamo Bay had F-91W watches. The F-91W, apparently, is a watch of choice among those which the U.S. considers its top enemies.

Why? A few reasons. First, it can be used as the timing device for an improvised explosive device, or IED. Per the documents reviewed by the Guardian, “the Casio was known to be given to the students at al-Qaida bomb-making training courses in Afghanistan at which the students received instruction in the preparation of timing devices using the watch.” That makes sense — it’s cheap, resilient, and mass produced; you can train a person on the watch and they can procure one pretty much anywhere. Second, as another Guardian article reported, there’s a religious reason: “according to testimony given by one prisoner, the model was useful because it was water-resistant: Muslims wash their arms up to their elbows before prayers.” And finally, as the second Guardian article explains, you can set a really long countdown timer on the Casio — it is “programmable up to 23 hours, 59 minutes and 59 seconds, [so] it allows the bombers to put ample distance between themselves and their targets.”

There are probably dozens of other digital watches which are similarly cheap and have the features noted above, but for reasons of practicality (using one model means fewer variables to account for and greater synchronization) or historical accident, Casio watches, especially the F-91W, have become the brand of choice for a certain group of malfeasance. That may change, however, as law enforcement is not only aware of the proclivity, but is using it to identify suspects. Just this week, a New York City college student was charged with “conspiring to support ISIS,” in the words of the New York Times. As the New York Post reports, one of the pieces of evidence that investigators cited in bringing these charges came from his Internet search history: “On May 10, ‘[the suspect] performed Internet searches for the terms ‘watch,’ ‘casio,’ and ‘vacuum’. . . these searches reflect [the suspect]’s efforts to identify and obtain components required to create an explosive device.’”

Bonus Fact : The image above of the F-91W comes from Casio’s official website, which you can visit here. If you browse through their site — check out this page and this one, too — you may notice something curious: in every case, the time is 10:58 and 50 seconds. That’s not a coincidence, but it’s not entirely meaningless, either. As the New York Times reported, many watch ads feature watches set to the time 10:10. That’s because when using an analog watch face, the watch brand/logo is typically under the 12. Putting the hands at 10 and 2 — 10:10, if the hour hand is pointing at the 10 — frames the brand marks nicely for the advertisements. But as digital watches have become increasingly common, some brands, like Casio, have shifted away from 10:10 and toward a minute (and second, in this case) of their own. According to one (questionable) source, it’s not just a branding move; Casio chose 10:58:50 because “it optimally demonstrates the LCD’s capabilities and takes up the most screen real estate.”

From the Archives : Bid Laden: How the U.S. employed a toy to fights Al-Qaeda.

Take the Quiz : How well do you know your digital watch faces? The quiz is easy — until you hit the secret bonus question (which isn’t terribly hard but it’s not that easy, either).