WASHINGTON, June 18 (Reuters) - The U.S. Supreme Court on Monday made it easier for prosecutors to use expert testimony about DNA laboratory reports at trial without allowing defendants to confront and question the forensic analysts involved in the tests.

By a 5-4 vote, the high court refused to add to a string of decisions since 2004 that have broadly interpreted the constitutional rights of criminal defendants to confront witnesses against them. The case involved a sexual assault in 2000 in Chicago for which the defendant was convicted and sentenced to life in prison.

The case had been closely watched because it could have had significant impact for prosecutors, law enforcement agencies and crime labs nationwide.

The Supreme Court's ruling was narrow, concluding that the expert testimony in the case from Illinois did not violate the U.S. Constitution. The dispute produced a splintered rationale that makes it difficult to predict how broadly the decision will affect future cases.

In the decision written by Justice Samuel Alito, the court said that the limited DNA testimony at issue in the case did not fall within the protections of the Constitution's "Confrontation Clause" guaranteeing that criminal defendants "be confronted with the witnesses against him."

Justice Elena Kagan, writing for the dissenters, said the ruling significantly confused this area of criminal law.

"Before today's decision, a prosecutor wishing to admit the results of forensic testing had to produce the technician responsible for the analysis," she wrote. "But that clear rule is clear no longer."

The Supreme Court ruled in 2009 that defendants have the right to cross-examine the analyst who prepares DNA, blood, ballistic and other laboratory reports introduced at trial.

The court then ruled in 2010 that the defendant must be allowed to question the lab analyst who did the test, not a colleague or supervisor.

The Supreme Court did not overrule those past decisions, but Monday's ruling casts a cloud over them.

Monday's case tested situations in which the DNA report has not been introduced as evidence at trial, but was used as the basis for an opinion by an expert witness.

Illinois was backed by more than 40 states in arguing that scientific experts should be allowed to testify about DNA data without having to call the analyst involved in the test.

The defendant in the case, Sandy Williams, was arrested on an unrelated charge and he provided a DNA sample for the state police lab database. A forensic analyst later testified at his trial about the procedures she had used to get his DNA sample and how she had entered the information into the database.

Police also recovered DNA material from the victim. That was analyzed by Cellmark Diagnostic Laboratory in Maryland, but the lab report was not introduced as evidence at trial and no one from the lab testified about it.

The DNA expert who did testify said she compared the two samples and offered her opinion that they matched. She said her testimony was partly based on Cellmark's testing procedures.

Williams was convicted on sexual assault charges, among other offenses, and sentenced to life in prison. The Illinois Supreme Court upheld his conviction. Williams appealed to the U.S. Supreme Court, arguing his constitutional rights had been violated.

The Supreme Court upheld the decision of the Illinois Supreme Court. "We conclude that there was no Confrontation Clause violation in this case," Alito wrote, noting that the expert referred to the testing only to explain the assumptions on which her opinion rested.

Joining Kagan in dissent were Justices Antonin Scalia, Ruth Bader Ginsburg and Sonia Sotomayor.

The Supreme Court case is Williams v. Illinois, No. 10-8505.

(Reporting by James Vicini; Additional reporting by Joan Biskupic)

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