Security is being stepped up across the country today after the release of a new audiotape in which Osama bin Laden threatens to attack the U.S. homeland -- though the government is not elevating the national alert level.
The new security measures are being taken as ABC News learns that al Qaeda Web sites have posted messages saying yet another tape is about to be released -- this one from al Qaeda's No. 2 man, Ayman al-Zawahiri, who apparently escaped a missile attack that targeted him in Pakistan a week ago. The Web sites say he will mourn his colleagues who were killed in that attack.
The bin Laden tape, declared authentic by the CIA, was broadcast Thursday by the Arabic news channel Al Jazeera. It was scratchy, and bin Laden's voice sounded weak -- less robust than on his last audiotape a year ago.
But there was nothing weak about what he had to say.
"The operations are under way," he said in a translation of the tape, which was in Arabic. "And you will see them inside your own home as soon as they are finished, God willing."
"He has renewed his threats against the United States," said FBI Assistant Director John Miller, who once interviewed bin Laden as a reporter for ABC News. "He has renewed his threats to have an attack on U.S. soil."
On the tape, bin Laden claimed that U.S. security measures were not difficult to penetrate and that al Qaeda was winning the battle in Iraq and Afghanistan. He also boasted of attacks on European capitols as proof of al Qaeda's strength.
"We have heard messages from al Qaeda and bin Laden where they have promised attacks and attacks have come to fruition," Miller said. "We've also seen tapes where they have promised attacks and then nothing has happened. That's the reason that we take every communication seriously."
Bin Laden's tough talk was followed by an offer of a truce with the United States.
"We have no objection to a long-term truce with you," he said, "based on fair conditions that we would fulfill."
White House spokesman Scott McClellan said it was a sign of weakness.
"We do not negotiate with terrorists," McClellan said. "We put them out of business."
Richard Clarke, a former White House security adviser who is now an ABC News consultant, told "Good Morning America" that bin Laden's "truce" offer was nothing more than a repackaging of his demand that the United States withdraw from all Arab countries.
"That's not a real truce offer. That's just his definition of victory for him," Clarke said. "If there is a major attack in the United States, he'll be able to say, 'Look, we gave the United States a chance … and they didn't take the offer.' "
Clarke told ABC News Radio that sensitive targets in the United States remained vulnerable to attack.
"If they were smart, and if they planned, and if they took advantage of our many vulnerabilities in this country that we still haven't addressed, they could do a lot of damage," he said.
Clarke added that an attack on a gas or chemical plant, for instance, might leave 17,000 Americans dead.
The Department of Homeland Security said it had no plans to raise the national security threat level. But precautions were being taken.
In response to the new tape, Homeland Security officials are sending out a bulletin to 18,000 police agencies telling them to review all their intelligence. Government sources told ABC News that local officials were being warned to pay attention to mass transit -- including subways, commuter trains and metro buses. Airports, water-treatment plants, chemical plants, and facilities storing radioactive material are also of concern.
A recent 9/11 Commission report card identified major areas of weakness in the country's security, including cargo being loaded on airplanes, imported cargo on ships, and mass transit.
"When I get on an airplane, I am less worried about the passengers than what is going into the hold below," said Lee Hamilton, former congressman and vice chairman of the 9/11 Commission. "I worry about containers being shipped into this country. I worry more about rail security than I do air security."
The national terror threat level currently stands at yellow, or elevated. The government has raised the alert level to orange, signaling a high threat risk, seven times since the attacks of Sept. 11, 2001.
ABC News' Brian Ross contributed to this report.