The 5-to-4 decision invalidates a Massachusetts law that authorized the introduction of neutral, scientific tests without giving the defendant an opportunity to question the testing technician during the trial.
Now, instead of simply admitting such results as evidence, a technician must appear personally in court to satisfy the requirements of the Sixth Amendment's confrontation clause.
The decision continues an important trend at the high court reinvigorating the right to confront one's accusers. The move began with a 2004 decision called Crawford v. Washington.
Justice Antonin Scalia has emerged as the champion of this newly bolstered right. Writing for the majority on Thursday, he echoed the 2004 decision, saying that the confrontation right is designed to guarantee that evidence is tested in the "crucible of cross-examination."
"Dispensing with confrontation because testimony is obviously reliable is akin to dispensing with jury trial because a defendant is obviously guilty," he wrote.
In an impassioned dissent, Justice Anthony Kennedy accused the majority of casting away 90 years of settled precedent embraced by 35 states and six federal courts of appeals.
"It is remarkable that the court so confidently disregards a century of jurisprudence," Justice Kennedy wrote. "We learn now that we have misinterpreted the confrontation clause ... for the first 218 years of its existence."
Kennedy and the dissenting justices warn that the court's holding will result in uncertainty and disruption of the criminal justice system. He says there aren't enough forensic experts to conduct the necessary tests and still be available to testify at trial.
"Guilty defendants will go free, on the most technical grounds, as a direct result of today's decision, adding nothing to the truth-finding process," Kennedy wrote.
Scalia counters that cross-examination will help deter fraudulent analysis, weed out incompetent technicians, and help prevent bias by experts. "A forensic analyst responding to a request from a law enforcement official may feel pressure – or have an incentive – to alter the evidence in a manner favorable to the prosecution," Scalia wrote.
Scalia dismissed the "dire" predictions of the dissenting justices. He said many states have already adopted similar measures and there is no evidence the criminal justice system has ground to a halt.
The decision comes in the case of Luis Melendez-Diaz, who was arrested, charged, and convicted of cocaine distribution and trafficking.
The arrest took place outside a Kmart store in Boston's Dorchester neighborhood. Mr. Melendez-Diaz and two other men were taken to the police station. Upon arrival, police discovered 19 plastic bags containing a white powdery substance in the back seat.
State lab tests identified the powder as cocaine. The tests were introduced as evidence at trial. Melendez-Diaz was convicted.
On appeal, his lawyer argued that the defendant had not been given a chance to cross-examine the technician who performed the tests. The lawyer cited the Supreme Court's 2004 decision in Crawford v. Washington.