U.S. Supreme Court Holds Defendants Have Right to Question Forensic Witnesses


On June 25, 2009, the United States Supreme Court held that prosecutors cannot use forensic reports as evidence in a criminal trial without giving the defendant a chance to question the analyst who prepared the report. The decision reaffirms the Sixth Amendment right of criminal defendants to confront witnesses against them.

The case, Melendez-Diaz v. Massachusetts, No. 07-591 (U.S. June 25, 2009), involved the 2001 conviction of Luis Melendez-Diaz for distributing and trafficking cocaine. At trial, the prosecution introduced several bags containing a white substance alleged to belong to the defendant, along with certificates stating the substance was cocaine. The defense requested the chance to cross-examine the forensic analyst who prepared the certificates, but the judge allowed the evidence and Mr. Melendez-Diaz was convicted.

Mr. Melendez-Diaz appealed, arguing that admitting the certificates without giving him a chance to question the analyst violated his Sixth Amendment right to confront witnesses against him. The Supreme Court agreed with Mr. Melendez-Diaz. In a decision by Justice Scalia, the Court reversed the Massachusetts courts, holding that because the analyst who prepared the certificate acted as a “witness” for the prosecution, Mr. Melendez-Diaz had the Sixth Amendment right to question the analyst in court.

The ruling highlights the importance of vigorous cross-examination of forensic analysts, who may be biased, fraudulent, or incompetent. “Forensic evidence is not uniquely immune from the risk of manipulation,” wrote Justice Scalia. Laboratories share close ties with law enforcement agencies, which may cause forensic examiners to “feel pressure – or have an incentive – to alter the evidence in a manner favorable to the prosecution.” There is no shortage of documented cases involving fraudulent forensics, such as where analysts reported results of tests that were never performed.

“Confrontation is designed to weed out not only the fraudulent analyst, but the incompetent one as well,” the Court wrote, citing “[s]erious deficiencies” found in the forensic evidence used in criminal trials, from fingerprint to toolmark and firearms analysis. A recent report from the National Academy of Sciences concluded that the forensic science system “has serious problems that can only be addressed by a national commitment to over-haul the current structure that supports the forensic science community in this country.” Justice Scalia, who rarely votes in favor of criminal defendants, relied on the widespread agreement in the legal community that “our system produces erroneous convictions based on discredited forensics.”

Requiring prosecutors to put forensic witnesses on the stand will allow defendants to challenge the accuracy of forensic testing by examining analysts about their knowledge, training, and methodology; provided, of course, that the defendant is represented by a competent lawyer who vigorously challenges forensic witnesses.