At a conference of law-enforcement forensics officials last week, someone asked David Miles what would happen if Apple Inc. tried to ruin his business. Mr. Miles heads an Atlanta startup called Grayshift LLC that sells a $15,000 iPhone-unlocking box to police and other authorities in the U.S. The device’s popularity has contributed to what some forensics investigators say is a golden age of iPhone investigations and led the conference attendee to ask what Grayshift could do if Apple tried to block it. “That’s the question everyone’s asking,” Mr. Miles said to muted laughter, before returning to a demonstration of how his device, called a GrayKey, could break into an iPhone and download nearly all of the data available on the device.