The warning from an anonymous tipster to California authorities quickly crackled across the nation to Boston: A group of people were on route to Massachusetts and planning a terror attack.

Patrols were increased at Logan International Airport (search) and the city's transit system. Vehicles were searched in underground parking garages. Gov. Mitt Romney (search) skipped President Bush's inauguration to return to Boston.

All this for what authorities now say was an act of revenge by an immigrant smuggler allegedly trying to get back at people who failed to pay him.

The apparent hoax has renewed scrutiny on the choices that authorities must make between deciding when to warn the public and when to investigate quietly without broadcasting the threat.

"Every day there comes to the various agencies within the U.S. government hundreds — thousands — of reports of everything from Martians having landed in Nevada to someone who just had a conversation with Elvis to terrorists coming with a nuclear bomb to Boston," said Graham T. Allison (search), an assistant defense secretary in the first Clinton administration.

"It's one of those situations where you're kind of damned if you do and damned if you don't," said Allison, now a professor of government at the John F. Kennedy School of Government (search) at Harvard University.

The tip that set Boston on edge was called in to the California Highway Patrol on Jan. 17. The caller claimed that four Chinese nationals and two Iraqi nationals entered the United States from Mexico and were awaiting a shipment of nuclear material that would follow them to Boston. The implication was that the group was plotting to detonate a radioactive "dirty bomb."

The list of possible terror suspects later grew to 16, although authorities continued to hedge, saying the tip was uncorroborated and unsubstantiated.

U.S. Attorney Michael Sullivan said the vast majority of anonymous tips received by authorities are not made public in an attempt to prevent panic. But authorities sometimes go public with a threat, he said, out of a concern for public safety or if they believe the public could help in the investigation.

Matthew Evangelista (search), a professor of international relations at Cornell University, said warning the public about a threat before it is thoroughly investigated can cause undue panic and may even cause people to be desensitized about the threat of terrorism.

"I think it breeds a kind of cynicism on the part of the public. ... People become maybe less willing to believe the threats when they are actually real," Evangelista said.

"The other risk it creates is a climate of suspicion, especially if people are identified by racial or ethnic characteristics. I think that's a danger officials need to think about when they issue these kinds of alerts."

In Boston, federal authorities went public after state and local law enforcement agencies took steps to increase security, and reports about the threat were leaked to the media.

The FBI released the photographs of the four Chinese nationals and later released the names, some passport numbers and possible birth dates for nine other Chinese and one Hispanic man they said were wanted for questioning. No information was released about the two Iraqis who were said to have been involved.

"It became apparent that the information was already in the public domain and there was significant risk that without making some kind of public statement, there was considerable risk of this being blown out of proportion and causing panic," Sullivan said.

Sullivan noted that authorities were careful to emphasize from the beginning that they had been unable to corroborate the claims of the tipster.

"It's a constant challenge of the part of law enforcement because they have to respond as if each of these are potential real threats even though you recognize that the vast majority of them are going to be washed out," he said.

"The biggest fear you have if you didn't address it as a real threat is that you're going to miss an opportunity to prevent [something] from happening."