(1) "A prosecutor who does not appreciate the perils of using rewarded criminals as witnesses risks compromising the truth-seeking mission of our criminal justice system. Because the Government decides whether and when to use such witnesses, and what, if anything, to give them for their service, the Government stands uniquely positioned to guard against perfidy. By its action the Government can either contribute to or eliminate the problem. Accordingly, we expect prosecutors and investigators to take all reasonable measures to safeguard the system against treachery."
(2) "A cooperating criminal is far more dangerous than a scalpel because an informer has a mind of his own, and almost always, it is a mind not encumbered by the values and principles that animate our law and our own Constitution. An informer is generally motivated by rank and frequently sociopathic self-interest and will go in an instant wherever he perceives that interest will be best served. By definition, informer-witnesses are not only outlaws, but turncoats. They are double crossers, and a prosecutor not attuned to these unpleasant truths treads without cleats on slippery ice. In a moment, a prosecutor can effectively go from prosecutor to the object of an investigation, with chilling consequences. Moreover, an informer even apparently fully on board can commit perjury, obstruct justice, manufacture false evidence, and recruit other witnesses to corroborate his false stories. After 40 years in our justice system, I conclude that the greatest threat to the integrity of our justice process and to its truth seeking mission -- indeed, even to prosecutors themselves -- comes from informers poorly chosen for their roles and then carelessly managed and handled. "
United States v. Bernal-Obeso, 989 F.2d 331, 333-34 (9th Cir. 1993), cited in informant_trott_outline.pdf. -
"THE USE OF A CRIMINAL AS A WITNESS: A SPECIAL PROBLEM" by Stephen S. Trott, Senior Circuit Judge United States Court of Appeals for the Ninth Circuit.