ASHINGTON, May 5 — Awakening a teenage boy in the middle of the night, putting him in handcuffs while he is still in his underwear and taking him out of his home for questioning does indeed amount to an "arrest," the United States Supreme Court said today in setting aside his murder conviction.

The court sharply criticized the sheriff's office in Harris County, Tex., and by implication the state Court of Appeals for its handling of the case of the teenager, Robert J. Kaupp, who has been serving a 55-year sentence.

The case began with the disappearance of Destiny Thetford, 14, in suburban Houston in January 1999. Investigators determined that the girl had had a sexual relationship with her 19-year-old half-brother, Nicholaus Thetford, and suspicion soon focused on him.

Deputies questioned Mr. Thetford and his friend Mr. Kaupp, who was then 17 and was with Mr. Thetford the day the girl disappeared. Mr. Kaupp cooperated with investigators, according to court records, but Mr. Thetford failed three polygraph tests and eventually confessed to fatally stabbing his half-sister and hiding her body in a drainage ditch.

Mr. Thetford pleaded guilty to murder and was promised a term of no more than 50 years in return for testifying against Mr. Kaupp. Mr. Thetford told investigators that Mr. Kaupp had helped him with the crime at every step, an assertion he would later repeat at Mr. Kaupp's trial.

After trying but failing to get a warrant to question Mr. Kaupp, several officers went to his family's house at 3 a.m. on Jan. 27, 1999, and awakened the youth with a flashlight.

"We need to go and talk," one officer told the teenager, according to court records.

Whereupon Mr. Kaupp was led, handcuffed, in his underwear and shoeless, to a patrol car. He was driven to the spot where the victim's body had been found, then on to sheriff's headquarters for questioning. At headquarters, the handcuffs were removed and Mr. Kaupp was read his Miranda rights against self-incrimination.

At first, records show, he denied any involvement in the victim's disappearance. But after 10 to 15 minutes of questioning, he admitted having a role in the crime, while denying that he inflicted the fatal wounds.

It was only then, the investigators maintained, that Mr. Kaupp was placed under arrest. Moreover, they asserted, Mr. Kaupp could have left of his own accord any time before that.

The youth's lawyer moved to suppress his confession as the fruit of an illegal arrest. The state Court of Appeals ruled against him, affirming the conviction and concluding that no actual "arrest" had occurred until after the confession at sheriff's headquarters.

Today, the Supreme Court rejected the Texas court's finding. It did so in a short, unsigned "per curiam" opinion without even hearing arguments, indicating that the justices' found the case an easy one to decide.

"It cannot seriously be suggested that when the detectives began to question Kaupp, a reasonable person in his situation would have thought he was sitting in the interview room as a matter of choice, free to change his mind and go home to bed," the high court said.

The justices said that since Mr. Kaupp was indeed under arrest the moment he was awakened, regardless of the deputies' assertion to the contrary, the defendant's confession must be suppressed unless the authorities can make a stronger argument than it has so far for keeping it in evidence. If the confession is not admitted into evidence, prosecutors can try to convict Mr. Kaupp without it.

The decision in Kaupp v. Texas, No. 02-5636, can be read on the court's Web site: www.supremecourtus.gov.