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 department also sent out bulletins to state and local officials about routine steps they can take and new precautions after the largely botched car-bomb plot in Britain late last month.
As for additional precautions expected this summer, "It can change at a moment's notice," said Chertoff's spokesman, Russ Knocke.
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."
The Sept. 11, 2001, attacks hit the country's financial nerve center and symbol of capitalism. Since then, regulations have been tightened to better guard the financial system against abuse from terrorist financiers and others.
These agencies report a high level of vigilance but few if any specific changes because of the latest worries.
With 4,000 law enforcement officers, the Interior Department says it is keeping busy. It is charged with protecting one of every five U.S. acres. "We ask our employees always to be vigilant," spokeswoman Tina Kreisher said.
The officers have bolstered security along borders, at sites such as the Statue of Liberty, the Liberty Bell, Independence Hall and the Washington Monument, and at national assets such as Bureau of Reclamation dams and the roads that lead to them.
The Environmental Protection Agency is the lead for hazardous materials response and works closely with industry. Under a government mandate, chemical makers are taking stock of chlorine, anhydrous ammonia (the basic ingredient for most nitrogen fertilizers) and other "chemicals of concern" that could -- if stolen -- cause damage by release or explosion.
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.
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.
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. The agency has the authority to investigate the suspected tampering of the products it regulates -- a list that includes drugs, vaccines, blood, medical devices and food.
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.
There has been no overall change in protection measures for domestic military installations. Individual commanders have authority to increase inspections or patrols when necessary. Checks of traffic going into some Washington-area bases have gotten more rigorous this summer, for example.
Basic security at military installations has been at the next-to-lowest level for more than four years. Protective measures added in recent years include entry barriers, road closures, surveillance cameras and armed guards, and programs to encourage service members and their families to report signs of possible terrorist planning.
Military bases are acknowledged to be vulnerable, particularly those close to urban areas and civilian roadways. Naval bases have the added problem of securing waterfronts and surrounding waterways.