WIRED: How the Government Hides Secret Surveillance Programs by Louise Matsakis:
First described in government documents obtained by Reuters in 2013, parallel construction is when law enforcement originally obtains evidence through a secret surveillance program, then tries to seek it out again, via normal procedure. In essence, law enforcement creates a parallel, alternative story for how it found information. That way, it can hide surveillance techniques from public scrutiny and would-be criminals. A new report released by Human Rights Watch Tuesday, based in part on 95 relevant cases, indicates that law enforcement is using parallel construction regularly, though it’s impossible to calculate exactly how often. It’s extremely difficult for defendants to discern when evidence has been obtained via the practice, according to the report.