The Fourth Amendment not ambiguous? Really..? This is probably one of the most ambiguous parts of The Constitution. Take a look at it.
You may also want to revisit your history; the US has a long standing tradition of wartime surveillance of the enemy, both foreign and domestic, and while we’re not at war with a State per se, we’re at war nonetheless (whether we should or not is besides the point).
This tradition of intel collection goes back to our colonial times and is a practice as old as communications itself; during the Revolutionary War George Washington made frequent use of his Army's collection capabilities; he directed the interception of British mail, the opening of such documents and the exploitation of their data. You may argue that this was directed at a foreign power and that’s true, but such logic has extended to other cases; during the Civil War the Union wiretapped the Confederate’s telegraphs and I remind you, without a warrant and while targeting US persons.
Even in more recent times such as WWI, Wilson authorized the US military to intercept telegraph, telephone and cable communications headed in and out of the US (sound familiar?); he derived this authority from The Constitution and from Congress’ declaration of war, an authorization to use military force that did not explicitly include language related to SIGINT collection methods.
This was once again done in WWII after Pearl Harbor and one can see how such protocols exist today, in a more technologically advanced manner through the NSA’s (and other's) various programs.
You may not like the truth for it is counterintuitive to what you believe, but the truth it is nonetheless…