ASHINGTON, Oct. 15 — When the police arrived at an apartment in Las Vegas in July 1998 to execute a warrant to search for drugs, they knocked on the front door and called out, "Police search warrant!" But LaShawn L. Banks, the apartment's resident, was taking a shower and did not hear them.

After waiting 15 or 20 seconds, the officers used a battering ram to knock down the door. Entering the apartment, they found crack cocaine, three guns and $6,000.

Whether the officers should have waited longer was the question in a Supreme Court argument on Wednesday. The justices' answer could clarify a particularly murky area of criminal law, how the "knock and announce" requirement that the court has identified as a hallmark of a reasonable search of a private home is supposed to work in actual police practice. Assuming that the police do knock and announce their presence, what happens next?

The case is an appeal by the federal government of a ruling by the United States Court of Appeals for the Ninth Circuit, in San Francisco, which found the search unconstitutional and the evidence therefore inadmissible at Mr. Banks's trial on federal narcotics and weapons charges.

Without saying exactly how long the police should have waited for Mr. Banks to open the door, the appeals court said the 15- to 20-second interval was not long enough, given that there were no special circumstances involved in executing the week-old warrant to justify the nearly instantaneous destruction of property.

David B. Salmons, an assistant solicitor general arguing the government's appeal, said the question was whether the officers' entry was reasonable under the "totality of the circumstances," a test he called the "hallmark" of the Supreme Court's approach to applying the Fourth Amendment's prohibition on unreasonable searches. Mr. Salmons called the Ninth Circuit's approach rigid and insufficiently flexible, adding that 15 to 20 seconds "is a substantial amount of time" to wait, given how easy it is to dispose of drugs behind a closed door.

Randall J. Roske, arguing for Mr. Banks, conceded that the "modern marvel of indoor plumbing does give the opportunity to destroy evidence."

But Mr. Roske told the justices that it also provided the opportunity to take a shower in the privacy of one's home, as Mr. Banks was doing when members of a joint police-Federal Bureau of Investigation task force burst in.

"What does a shower have to do with it?" Justice Antonin Scalia asked. "Is it your contention that a reasonable time is how long it takes to complete a shower?"

Mr. Roske started to answer, "We don't know how long Mr. Banks would have continued showering."

Justice Scalia interrupted. "We don't know and we don't care," he said.

Earlier in the argument, various justices appeared skeptical of aspects of the government's argument, particularly Mr. Salmons's insistence that whether the police destroy a door or enter through an unlocked one should have nothing to do with the determination of whether a particular length of time before entering was reasonable.

"You're telling me that it's just as reasonable to have to wait 15 seconds when entry requires walking in and when entry requires bashing the door down?" Justice Scalia said to Mr. Salmons in an incredulous tone. "That doesn't reflect the totality of the circumstances."

Whatever questions they had about the government's case, the justices appeared singularly unpersuaded by Mr. Roske's defense of the Ninth Circuit decision. Those whose votes he needed to have a chance of prevailing gave him little encouragement.

"Why isn't 15 to 20 seconds ample time to get to the door in a small apartment?" Justice Ruth Bader Ginsburg asked, noting that a telephone rings two or three times in that interval and that "usually people pick up the telephone after the second ring."

Justice David H. Souter observed that although "there might be all sorts of reasons" why a particular suspect could not go to the door — "They might be asleep or sick in bed with a virus" — the question was what time was reasonable "under the circumstances known to the officers."

Several justices emphasized the ease of destroying evidence when drugs were involved.

A federal law authorizes officers to break down a door if they are "refused admittance" after announcing their presence. But the justices seemed to agree that the statute did not literally depend on an explicit refusal.

This case, United States v. Banks, No. 02-473, was the latest in a series of Supreme Court cases examining the knock and announce requirement which, as Mr. Roske emphasized, has deep roots in English common law. In a decision in 1997, the court unanimously rejected a rule that the Wisconsin Supreme Court had adopted to suspend the knock and announce requirement for any drug search.

In 1995, another unanimous decision held that whether the police announced their presence before entering a home formed part of the evaluation of whether the ensuing search was constitutionally reasonable. That decision clearly placed the knock and announce rule on a constitutional footing for the first time. But in also holding that the concept of reasonableness itself meant that there could be exceptions, that decision, Wilson v. Arkansas, left many questions unanswered.