A Nation On Guard

The U.S. government was trying to strike a balance between vigilance and calm Thursday, CBS News Chief White House Correspondent John Roberts reports, amid mounting evidence that terrorists may be planning a year-end attack on American soil.

Citizens are being warned against opening any unexpected packages bearing a postmark from Frankfurt, Germany. Millions of parcels bound for holiday destinations are now being x-rayed at the Postal Service's 11 airport gateways into the country.

The threat came from an unsubstantiated telephone tip. But with America gripped by a serious case of the jitters, it was enough to prompt the United States Postal Service into action.

According to the USPS's Dan Mikhalo, "In this atmosphere, with some of the other terrorist activity going on, we're not treating it lightly. We're looking at this very seriously."

This latest warning comes as more is learned about Ahmed Ressam, who was indicted Wednesday on charges of smuggling explosives into the country. Intelligence sources told CBS News that Ressam had ties to associates of terrorist financier Osama Bin Laden, but it's not yet known if he was directed or financed by Bin Laden himself.

Manhunts are under way for up to three people suspected of ties to Ressam and for a California man said to have ties to bin Laden.

Authorities know that Ressam had planned to visit Seattle; the city has announced that a part of its New Year's celebrations -- a ceremony to burn papier mache sculptures while four dark figures on horses rode around -- would be canceled. There were concerns that it could have been mistaken to represent the Four Horsemen of the Apocalypse.

Discussing the possibility of a millennium terrorist attack with CBS News, U.S. ambassador Michael Sheehan, the State Department's coordinator for counterterrorism, said consultations regarding the issue have been going on for several months. "What has changed," he explained, "is we now have specific threats that we have uncovered or are in the course of investigation, which have heightened the sense of awareness of our government."

Sheehan said that reports of specific threats against New York, Washington, and Seattle, "have not been corroborated or substantiated. And right now, there is no specific or credible threats against any sites in the United States."

He thought there was no link between the State Department's latest reaction and the criticism that was leveled at federal agencies for not picking up on threats against the American embassies in Eastern Africa a year-and-a-half ago. "We in the government agencies around this town (Washington) take our responsibilities very seriously," he said. "We take a look at all the specific threats and information that we have, try to make judgments on what is credible and specific, and share that information with the American public."

Meanwhile, at San Francisco International airport hursday, CBS News Correspondent John Blackstone reports, tightened security seemed like an attack on the holiday spirit as packages meant to be unwrapped Christmas morning were instead ordered opened at security checkpoints.

"It's a little inconvenient," said traveler Jim Duncan, "But hey, as a long as I get from here to there safe, that's all I care about."

For holiday travelers across the U.S., fears of terrorism added one more complication at already congested airports. With more bags and more passengers being looked at more carefully, the usual long holiday lines grew even longer. At most airports the extra security starts at the curb, with parking rules strictly enforced.

Newark airport has positioned plenty of uniformed police among the holiday travelers. At San Francisco International, eight bomb-sniffing dogs are patrolling the terminal this week.

But part of the holiday travel story is good news. In San Francisco and across much of the country, unusually clear dry weather has meant few delayed flights. It seems as if holiday travelers will put up with almost any security measures as long as their plane leaves on time.

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