Writing for Reason Radley Balko gives an overview of our corrupt crime lab, as well as similar experiences in quite a few other states.

He gets at the core of the problem: “Forensic science in America is corrupted by a fundamental conflict of interest. In far too many states, crime labs fall under the auspices of law enforcement, usually reporting to the state attorney general. A forensic analyst’s real aim should be the follow the science, even if results prove disappointing to bosses who are trying to secure convictions. But the pressure from prosecutors, even when it’s not overt (which it often is), produces bias even in the work of the most fair-minded analysts.”

I recommend the whole piece.