ATAKYA, Turkey — Abu Ali, a commander of a moderate Syrian rebel group in northern Syria, proudly shows a black laptop partly covered in dust. “We took it this year from an ISIS hideout,” he sayAbu Ali says the fighters from the Islamic State of Iraq and al-Sham ISIS, which have since rebranded themselves as the Islamic State, all fled before he and his men attacked the building. The attack occurred in January in a village in the Syrian province of Idlib, close to the border with Turkey, as part of a larger anti-ISIS offensive occurring at the time. “We found the laptop and the power cord in a room,” he continued, “I took it with me. But I have no clue if it still works or if it contains anything interesting.”As we switched on the Dell laptop, it indeed still worked. Nor was it password-protected. But then came a huge disappointment: After we clicked on “My Computer,” all the drives appeared empty.Appearances, however, can be deceiving. Upon closer inspection, the ISIS laptop wasn’t empty at all: Buried in the “hidden files” section of the computer were 146 gigabytes of material, containing a total of 35,347 files in 2,367 folders. Abu Ali allowed us to copy all these files — which included documents in French, English, and Arabic — onto an external hard drive.The laptop’s contents turn out to be a treasure trove of documents that provide ideological justifications for jihadi organizations — and practical training on how to carry out the Islamic State’s deadly campaigns. They include videos of Osama bin Laden, manuals on how to make bombs, instructions for stealing cars, and lessons on how to use disguises in order to avoid getting arrested while traveling from one jihadi hot spot to another.