If police ask who you are, do you have to say?

The American tradition of liberty is such that citizens generally do not have to justify their existence by producing government-issued identity papers whenever ordered to do so. This is why moves to establish a national identity card have never gotten off the ground.

But what happens when a police officer believes you might be involved in a crime and asks your name as part of the investigation? Do you have to answer?

That is the question before the US Supreme Court Monday, as the justices consider whether a Nevada law requiring suspects to identify themselves whenever requested by police violates constitutional protections of privacy and freedom from self-incrimination. The case is significant because it gives the court a chance to more closely define how deeply law-enforcement officials may intrude into private lives.

If the court establishes a bright-line rule barring police from forcing such disclosures, it will highlight a new focus by the justices on individual liberty. If, on the other hand, the court upholds the Nevada law, it could prompt other states and the federal government to adopt similar tactics amid heightened concern about possible terrorist activities within the US.

The case arises at a time when it has become virtually impossible to board a commercial airliner for a domestic US flight without first producing some form of government-issued ID.

Under Nevada law, a citizen must reveal his or her name to a police officer who has reasonable suspicion that the person might be involved in a crime. Even if the suspect is innocent, the mere act of refusing to identify oneself is - itself - a crime.

Analysts say the law creates a legal irony. If the police officer possessed enough evidence to place the suspect under arrest, the suspect would be given a Miranda warning that he or she had the right to remain silent. But if the police officer possessed only reasonable suspicion - not the higher standard of probable cause needed to justify an arrest - a suspect could be arrested and convicted merely for refusing to identify himself.

"Why should a criminal have more rights than an innocent person?" asks Harriet Cummings, a Nevada public defender who is part of the team challenging the law.

The issue stems from the case of Larry Hiibel, who in May 2000 was arrested and convicted of hindering a police investigation solely because he refused to identify himself to a deputy sheriff.

The incident took place at a roadside in Humboldt County, Nev. A concerned citizen called the sheriff's department to report a man striking a female passenger in a pickup truck. When police arrived, the sheriff's deputy found Mr. Hiibel standing outside the truck with his minor daughter sitting inside.

Based on the earlier tip, the deputy had reasonable suspicion to believe Hiibel was involved in striking the girl. To further his investigation, the deputy asked Hiibel his name. Hiibel refused to answer, saying he didn't believe he had done anything wrong.

Eleven times the deputy asked Hiibel for his name. After the 11th refusal, the deputy placed Hiibel under arrest for resisting a police officer. Hiibel was convicted of violating Nevada's disclosure law. A state appeals court and a divided Nevada Supreme Court upheld his conviction.

In urging the US Supreme Court to overturn his conviction, Hiibel and his lawyers argue that police are free to ask a suspect any questions they want, but the suspect does not have to answer.

A law that can send someone to jail for refusing to speak violates both Fourth Amendment privacy protections and Fifth Amendment guarantees against being compelled to make incriminating statements, they say. "It is inimical to a free society that mere silence can lead to imprisonment," writes James Logan, a Nevada public defender and one of Hiibel's lawyers, in his brief to the court.

The Nevada Attorney General's Office counters that the state's interest in investigating crimes outweighs Hiibel's interest in keeping his identity private. "A person does not have a Fourth Amendment right to refuse to identify himself when detained on reasonable suspicion," says Conrad Hafen, senior deputy attorney general, in his brief. Asking someone's name is a minimal intrusion, Mr. Hafen says. Rather than forcing a suspect to make incriminating statements, repeating one's name does not provide evidence of a crime but merely assists an investigation, he says.

"Though the name may link the person to an outstanding warrant, it does not compel the person to inform the officer that he has an outstanding warrant," Hafen says. "A person's name is more like a fingerprint, voice exemplar, or handwriting analysis. It is used by law enforcement to identify the person."

Experts in electronic privacy disagree. "A name is now no longer a simple identifier: it is the key to a vast, cross-referenced system of public and private databases, which lay bare the most intimate features of an individual's life," says Marc Rotenberg, in a friend-of-the-court brief filed by the Electronic Privacy Information Center.

The US Supreme Court in prior cases has upheld pat-down and vehicle searches by police operating on reasonable suspicion. Supporters of the Nevada law say the act of asking and revealing one's name is far less intrusive than being frisked.