A fractured Supreme Court on Monday took a tentative step away from its recent decisions that said criminal defendants have the right to confront technicians and analysts who prepare crime-lab reports used against them.

The 5 to 4 decision upheld the rape conviction of an Illinois man. But it exposed the divisions on a court that is trying, as Justice Elena Kagan noted, to apply the 18th-century right to confront one’s accusers with 21st-century evidence such as DNA testing.

In the end, the justices split in ways that scramble the court’s usual ideological divide.

Justice Samuel A. Alito Jr., joined by two of his fellow conservative justices and one liberal, gave prosecutors more leeway in presenting laboratory reports without making analysts testify to their accuracy.

Kagan dissented, joined by two fellow liberals and conservative Justice Antonin Scalia. She said the constitution’s confrontation clause was violated when a state expert testified about a DNA report even though no one from the company that prepared it was called to vouch for its accuracy.

The U.S. Supreme Court Building in Washington. (J. Scott Applewhite/AP)

Justice Clarence Thomas decided the outcome. He agreed with the Alito plurality to call the case in favor of Illinois prosecutors, but for reasons that none of the other justices endorsed.

Douglas Berman, an expert on criminal law at the Ohio State University law school, called the 98-page decision that was six months in the making “a bloody mess.”

But he said it was born of a genuine and “collective concern about modern innocence issues” and the best way to accommodate prosecutors’ worries about an overwhelmed criminal justice system.

The case began with the 2000 rape of a Chicago woman. Police gathered semen samples from the victim and sent them to the Illinois State Police lab. That lab sent the samples to Cellmark Diagnostics Laboratory, based in Germantown, Md., which prepared a male DNA profile.

An Illinois forensics expert, Sandra Lambatos, conducted a computer search and found that the profile matched that of Sandy Williams, who had been arrested on unrelated charges. The victim picked Williams out of a line-up.

At Williams’s trial in 2006, Lambatos testified as to how she made the match, but the Cellmark lab report itself was not introduced into evidence.

Alito said that Lambatos was free to testify about the process and Cellmark’s role and that Williams’s attorney had the opportunity to cross-examine her. She was not testifying to the truth of the Cellmark report, the justice said.

He noted that the purpose of the report “was to catch a dangerous rapist who was still at large, not to obtain evidence for use against” Williams.

The Obama administration and 42 states had urged the court to accept such a position, saying that to do otherwise would hamper the use of expert testimony common in criminal trials.

Kagan wrote in dissent that the decision diminished recent rulings that had revived the importance of the confrontation clause.

“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. “That was the result of not one, but two decisions this court issued in the last three years.

“But that clear rule is clear no longer.”

Thomas reasoned that the rape conviction should stand because the Cellmark report “lacked the requisite formality and solemnity” to be considered the kind of testimony subject to the confrontation clause. But his view did not attract the support of any of the other justices.

“Indeed, Justice Thomas’s approach, if accepted, would turn the Confrontation Clause into a constitutional geegaw — nice for show, but of little value,” Kagan wrote.

The case is Williams v. Illinois .

The decision came on a day when the court was packed with spectators and lawyers, all of them aware that the court’s most important decisions — such as those addressing President Obama’s health-care law and Arizona’s immigration crackdown— will come within the next 10 days.

Kagan acknowledged the public anticipation of those rulings when she announced that she had written one of Monday’s decisions, about whether a Michigan man could challenge the government’s decision to take land into trust on behalf of an Indian tribe.

“This is a case about sovereign immunity and prudential standing — maybe not what you’ve all come for today,” she said to laughter.

The court will issue more decisions on Thursday.