The Supreme Court ruled yesterday that police officers may not deliberately avoid warning suspects of their right to remain silent before beginning questioning, asserting that a law enforcement tactic of interrogating suspects twice -- before reading them their rights and then after -- undermines the familiar Miranda right.

The 5 to 4 decision affirms the rights of suspects not to speak to investigators and is intended to end what the court said was an increasing police practice of twice questioning suspects in the hope of eliciting a confession. The decision rejects what the court called "a police strategy adapted to undermine the Miranda warnings."

Writing for the majority, Justice David H. Souter said that a Missouri woman's statements about her involvement in a murder plot were inadmissible because police had arrested her at 3 a.m., elicited her confession and only then advised her of her rights. She subsequently incriminated herself again.

"The reason that question-first is catching on is as obvious as its manifest purpose, which is to get a confession the suspect would not make if he understood his rights at the outset," Souter wrote in his opinion, which was joined by Justices John Paul Stevens, Ruth Bader Ginsburg and Stephen G. Breyer. Justice Anthony M. Kennedy filed a separate concurring opinion.

"The sensible underlying assumption is that with one confession in hand before the warnings, the interrogator can count on getting its duplicate, with trifling additional trouble," Souter wrote.

The ruling came in one of two closely watched cases decided yesterday that clarify the court's landmark 1966 ruling in Miranda v. Arizona, which first established "the right to remain silent." The Justices upheld Miranda in 2000, declaring that the Constitution requires police to inform suspects of their right not to answer questions and have attorneys present before interrogations can proceed. But the 2000 decision was broad and did not address interrogation techniques or other specific components of Miranda.

In the other case yesterday, the justices ruled that prosecutors may use physical evidence against a suspect even if it was obtained by officers who had not given the suspect a Miranda warning.

Supporters of Miranda had been especially tracking the case about the questioning of suspects, Missouri v. Seibert, No. 02-1371. In the case, Patrice Seibert was convicted and sentenced to life in prison after her second confession was admitted into evidence.

Seibert had been concerned that she would be charged with neglect in the death of her severely disabled son, Jonathan, at the family's mobile home, because of bedsores on his body. Two of her teenage sons and two of their friends hatched a plan in Seibert's presence to burn the trailer. To make it seem that Jonathan, 12, had not been left alone, they left a mentally disabled youth, Donald Rector, 17, in the trailer before setting it afire. Rector died of smoke inhalation.

Richard Hanrahan, a police detective in Rolla, Mo., elicited Seibert's confession, took a 20-minute break, then read her rights to her before she confessed again. The Missouri Supreme Court threw out the confession and the state, joined by the Bush administration, appealed.

Hanrahan said that he had been trained to question before and after giving a Miranda warning by a "national institute." The International Association of Chiefs of Police said yesterday that it does not endorse the tactic and does not teach it.

In an interview, Steven D. Benjamin, a Richmond lawyer who is on the board of the National Association of Criminal Defense Lawyers, said the two-step process has become so common that it has a name in the criminal justice community: "interrogating outside Miranda."

"It is an institutional practice among police. It happens every day," Benjamin said.

Souter wrote that the court has no statistics on the practice. But he said that although it is not used "en masse" by police, it is taught in some police training programs and is clearly a tactic "of some popularity" among law enforcement.

Justice Sandra Day O'Connor wrote a dissent, joined by Chief Justice William H. Rehnquist and Justices Antonin Scalia and Clarence Thomas.

In the second case, U.S. v. Patane, No. 02-1183, the court ruled 5 to 4 that physical evidence gathered because of a suspect's statement made without a Miranda warning does not have to be excluded from a case.

In the case, a detective began advising Samuel Francis Patane of his rights when Patane, a convicted felon, was arrested. Patane interrupted the agent, saying that he already knew his rights, and then directed the agent where to find a .40-caliber pistol. Patane sought to have that evidence excluded from his trial for illegal possession of a firearm.

The Denver-based U.S. Court of Appeals for the 10th Circuit agreed that the gun should be excluded.

But the high court overturned that ruling. Writing for the court, Thomas said that excluding physical evidence derived from an unwarned statement would be an unlawful extension of Miranda. His decision was joined by Rehnquist and Scalia.

Kennedy wrote a concurring opinion, joined by O'Connor. Souter, Stevens and Ginsburg dissented; Breyer dissented separately.

"There is, of course, a price for excluding evidence, but the Fifth Amendment is worth a price," Souter, Stevens and Ginsburg said. "There is no way to read this case except as an unjustifiable invitation to law enforcement officers to flout Miranda when there may be physical evidence to be gained."

Civil liberties advocates said the decision ending the double questioning of suspects was extraordinarily important. But they said they are worried that the Patane ruling could help erode its impact by encouraging police to avoid Miranda if they need to find crucial evidence -- a position endorsed by the four dissenting members of the court.

"It's a very mixed message," said Christopher Dunn, associate legal director of the New York Civil Liberties Union and an expert on Miranda rights. "On one hand, the court has wisely closed the door on police strategies designed to induce improper confessions. On the other hand, the court may be encouraging similar strategies in cases where they care less about a confession and more about the evidence they can obtain through a coerced confession."