"The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized."
"papers, and effects" seems to cover internet communications to me (the closest analog available to the authors being courier mail of messages written on paper), but the secret courts so far seem to have disagreed: https://en.wikipedia.org/wiki/United_States_Foreign_Intellig...