Is This Legal?

I don't know, but it sure smells bad.  From August 6, 2013 Reuters:

WASHINGTON (Reuters) - A secretive U.S. Drug Enforcement Administration unit is funneling information from intelligence intercepts, wiretaps, informants and a massive database of telephone records to authorities across the nation to help them launch criminal investigations of Americans.

Although these cases rarely involve national security issues, documents reviewed by Reuters show that law enforcement agents have been directed to conceal how such investigations truly begin - not only from defense lawyers but also sometimes from prosecutors and judges.

The undated documents show that federal agents are trained to "recreate" the investigative trail to effectively cover up where the information originated, a practice that some experts say violates a defendant's Constitutional right to a fair trial. If defendants don't know how an investigation began, they cannot know to ask to review potential sources of exculpatory evidence - information that could reveal entrapment, mistakes or biased witnesses.

The problem is that recreating a case from sources not "fruit of the poisonous tree" encourages some great creativity in figuring out how to re-create those sources.  I'm prepared to tolerate considerable surveillance latitude for counterterrorism, because these are not cases that are generally not going to criminal trial.  We're trying to prevent losing cities more than get a conviction.  Once you extend this same latitude to criminal matters -- and especially relatively trivial criminal matters -- I get very, very uncomfortable.

24 was a great show, and it did convey some of the problems can happen when you have to decide between losing a city or cutting some corners.  These are horrifying decisions to make.  If they have to be made once in a blue moon to avoid terrorism attacks, I cringe but accept it.  When they get made weekly for thousands of cases a year?  That's far too scary.