You probably know the dilemma of the trolley problem: A runaway trolley is barreling down the track and will kill five innocent people in the way. You can pull a switch that will direct the trolley to a different track. But another man is standing on that second track, and pulling the switch will lead to his death.
It’s an excruciating choice: Do nothing and let five die, or pull the switch to save four lives but also take responsibility for one person’s death.
The F.B.I. faces a similar dilemma when its agents seize the computer servers of a child pornography site hosted on the Internet’s “dark web.” The F.B.I. has two difficult options for when to shut it down.