The mysterious visitor called himself Gary Host at first, then Grace Host, which he shortened for his made-up e-mail address to “ghost,” a joke apparently, perhaps signaling mischievousness — or menace. The intruder was lurking somewhere on the MIT campus, downloading academic journal articles by the hundreds of thousands.

The interloper was eventually traced to a laptop under a box in a basement wiring closet. He was Aaron Swartz, a brilliant young programmer and political activist. The cascade of events that followed would culminate in tragedy: a Secret Service investigation, a federal prosecution, and ultimately Swartz’s suicide.

But in the fall of 2010, Swartz was still a stranger in the shadows, and the university faced a hard question: How big a threat was the “ghost” downloader? And a harder one: What should be done about him?