HAT should Eunice Stone have done?
As she was having breakfast at a Shoney's restaurant in Calhoun, Ga., one day earlier this month, she said she overheard a conversation among the three men in the next booth that alarmed her.
So Ms. Stone, a nurse from Cartersville, Ga., called the police and said that she heard the men discussing terrorism plans. As a consequence, law enforcement officials closed down the Interstate highway known as Alligator Alley and detained the men for the better part of a day. The men, who were Muslim medical students on their way to a hospital in Miami, endured a thorough search of their cars for explosives and intense public scrutiny before they were released.
Ms. Stone has been praised for her alertness as well as criticized for what civil-liberties advocates said was paranoia. She has hired lawyers. Yet Ms. Stone was clearly heeding the call of government officials, from President Bush on down, who have asked citizens to be vigilant and report anything suspicious.
It is hard to know how to calculate the costs and benefits of encouraging tips like that of Ms. Stone's, said Cass R. Sunstein, a law professor at the University of Chicago.
"There are two things to balance," he said. "One is the likelihood that you'll get good information that will prevent harm. The other is that you will get people in trouble who don't deserve to be or that you will create a culture of surveillance."
It is not difficult to identify in hindsight tips that should have been made but were not. After Sept. 11, 2001, for instance, a federal Department of Agriculture official in Florida was criticized for failing to report a charged encounter in 2000 with Mohamed Atta, believed to be the ringleader of the terrorist attacks. Other people came forward after Sept. 11 with stories about disturbing encounters with the hijackers that they did not report.
Last week, after the arrests of six Arab-American men from Lackawanna, N.Y., who are accused of being members of a Qaeda cell, New York state established a toll-free hotline for reporting suspicious activity. Information from within the Yemeni community in Lackawanna seems to have played a role in the arrests.
James McMahon, the superintendent of the New York State Police, said people should err on the side of passing along information, even if it is based only on "instincts and intuition."
But is it possible to be too vigilant? Does the flood of resulting tips obscure or even reduce, as the boy who cried wolf learned, the truly valuable information?
Even Gov. George E. Pataki sounded a little wary about the hotline when he announced it, asking people to use it "responsibly and with common sense."
Civil libertarians said that may be asking too much of the public.
"We will find ourselves falling into anarchy if we ask ordinary people to play the role that only law enforcement officials can play properly," said Anthony D. Romero, the executive director of the American Civil Liberties Union.
But others say such comments exaggerate and overreach.
"Informing in a country where thought crimes are criminal is one thing," said Charles Fried, a law professor at Harvard. "Informing in a country where the only crimes are menaces to public security in a physical sense is very different."
Mr. Sunstein said there was no need for alarmism. "I think our culture is strong enough that the likelihood that this will create serious problems for our freedom is very low,"` he said.
Those upon whom suspicion is cast have little legal recourse. It is permissible to contact the authorities with leads based on misunderstandings or ill-founded suspicions.
Various legal claims may be available in theory, experts said, among them defamation, infliction of emotional distress and malicious prosecution. But they are not likely to succeed. In most states and in most situations, plaintiffs must prove not only that the tip was false but that the informer knew or suspected that at the time and meant to harm the subject of the tip.
In the current environment, said Prof. Rodney A. Smolla of the University of Richmond School of Law, "there is not much realistic exposure to liability, though it would depend on the pointedness of the tip." A purposefully false accusation of training pilots for terror missions, he said, might be treated differently from the voicing of more generalized suspicions.
It gets more complicated when suspicions result in fights and imprisonment, as in the case of two men of Indian descent who were jailed in Arkansas earlier this month after a Northwest Airlines flight attendant grew concerned about their trips to the airplane bathroom. They may be able to sue the airline and the government for malicious prosecution or false imprisonment.
"It all turns on the question of probable cause," said Richard D. Emery, a civil rights lawyer.
Ibrahim Hooper, a spokesman for the Council on American-Islamic Relations, said untrained civilians are more likely to engage in racial stereotyping.
"There is a real chill in the Muslim community about what will be taken as an innocent comment," Mr. Hooper said.
And not all tips are made in good faith, he said.
"We've had reports of cases where people use tips as payback on the job," Mr. Hooper said. "We see it in custody cases, where people make all kinds of allegations against others if they're Muslims."
But false accusations are a part of law enforcement, Mr. McMahon said, and that is where police investigative skills come in. "Whether it comes in as a tip, a 911 call or over a seven-digit number, you're going to get false reports," he said.
If there are to be errors, Mr. McMahon said, they should be on the side of too much information.
"There are probably going to be a lot of tips that come in baseless," he said. "I'd rather hear about it."