Washington National security officials worry about a possible attack against the United States in the months ahead even though the government's leading terrorism experts have not found concrete information about an imminent strike.
Homeland Security Secretary Michael Chertoff spoke this past week of his "gut feeling" that the nation faces an increased risk of attack this summer.
President Bush's instincts point in the same direction. "My head also tells me that al-Qaida's a serious threat to our homeland," he said at a news conference Thursday. "And we've got to continue making sure we've got good intelligence, good response mechanisms in place."
As early as this coming week, the administration is expected to release an unclassified version of a new National Intelligence Estimate - spy agencies' most authoritative type of appraisal - on al-Qaida's resurgence and the group's renewed efforts to sneak operatives into the United States.
A look at what the government says it is most worried about and what it is doing to thwart potential attacks:
Chertoff is asking people to be on watch for suspicious behavior or activities in transit systems or other public places. "When you see something, say something," he often says. That means picking up the phone to alert local authorities or federal law enforcement about anything out of the ordinary, such as a suspicious person, package or vehicle.
Just before the July 4 holiday, the Transportation Security Administration dispatched VIPR teams (Visible Intermodal Protection and Response) to airports and mass transit systems in Washington, Baltimore, New York, Boston, Philadelphia, Houston, Los Angeles and San Francisco.
They include canine teams, agency officers trained in behavior observation, additional air marshals, surface transportation security inspectors and local police.
Federal air marshals already had bolstered their presence on domestic and international flights since last August, when international authorities foiled a plot to blow up about 10 U.S.-bound jetliners coming from London. That stepped-up presence continues today.
The Treasury Department is keeping close watch for fresh clues on sources of financing for terrorist groups. Yet counterterrorism officials say that attacks do not have to be expensive. The Sept. 11 Commission estimated the 2001 attacks cost $400,000 to $500,000.
"By exploiting financial intelligence, the Treasury can map terrorist networks and reveal who is sending money to al-Qaida, Hezbollah and like-minded terrorist groups," department spokeswoman Molly Millerwise said. "These efforts allow us to detect and disrupt the flow of finances to terrorists, making it harder and riskier for them to store and move money."
Energy and nuclear
Nuclear power plants long have been viewed as a top target of terrorists and have tightened security since Sept. 11. But the latest concerns have not led to significant changes or alerts. "We are paying close attention to what the intelligence community is reporting and will act accordingly," Nuclear Regulatory Commission spokesman Eliot Brenner said.
At the Energy Department, which oversees the government's nuclear weapons facilities, including its national research labs, security requirements have been revamped since 2001, especially in the protection of nuclear materials.
Thousands of miles of oil and natural gas pipelines as well as refineries also have been regarded as potential terrorist targets. But with no specific threat, the industry's response to the latest concerns has been simply to remain cautious.
Food and drug
The Food and Drug Administration is helping foster the development and acquisition of vaccines, diagnostic tests and drugs that can be used against attacks including anthrax, botulism, radiological agents, smallpox and plague.
In a declared emergency, the FDA can authorize the use of unapproved medical products to diagnose, treat or prevent illnesses due to biological, chemical, radiological or nuclear attack.
With the FBI and the Agriculture and Homeland Security departments, the FDA has begun assessing various foods - from a list of roughly 30 - to determine their vulnerability to attack at various points in the production process. In 2005-06, for instance, the FDA visited yogurt, baby food, bagged salad and other producers.
The FDA considers foods processed in large batch sizes, or ingredients subsequently mixed with large amounts of product, the most vulnerable to terrorism.
The Agriculture Department is most concerned about devastating animal or plant diseases that could be introduced intentionally into the United States. These include avian influenza and others that could move from animals to humans.
The department has worked with farmers and shippers to educate them on prevention against tampering, asking them to make sure supplies are locked, for example, and asking truckers never to leave shipments unattended.
The department also is working with veterinarians to make sure they are knowledgeable about exotic diseases that may appear in animals.
Other concerns include the misuse of agricultural pesticides and the entry of suspicious people through U.S. border farms, which often are expansive and largely unprotected.