Law enforcement officers conducting a legal search have always been able to pursue evidence of other crimes sitting in “plain view.” Investigators with a warrant to search a house for drugs can seize evidence of another crime, such as bombmaking. But the warrant does not allow them to barge into the house next door.

But what are the comparable boundaries online? Does a warrant to search an e-mail account expose the communications of anyone who exchanged messages with the target?

Similarly, FBI agents monitoring wiretaps have always been obligated to put down their headphones when the conversation is clearly not about a criminal enterprise. It’s known as minimization, a process followed by intelligence and law enforcement agencies to protect the privacy of innocent people.

“It’s harder to do with e-mails, because unlike a phone, you can’t just turn it off once you figure out the conversation didn’t relate to what you’re investigating,” said Michael DuBose, a former chief of the Justice Department’s Computer Crime and Intellectual Property Section who now handles cyber-investigations for Kroll Advisory Solutions.