WASHINGTON — In the days immediately after the attacks of Sept. 11, 2001, members of President Bush’s war cabinet declared that it would be impossible to deter the most fervent extremists from carrying out even more deadly terrorist missions with biological, chemical or nuclear weapons.

Since then, however, administration, military and intelligence officials assigned to counterterrorism have begun to change their view. After piecing together a more nuanced portrait of terrorist organizations, they say there is reason to believe that a combination of efforts could in fact establish something akin to the posture of deterrence, the strategy that helped protect the United States from a Soviet nuclear attack during the cold war.

Interviews with more than two dozen senior officials involved in the effort provided the outlines of previously unreported missions to mute Al Qaeda’s message, turn the jihadi movement’s own weaknesses against it and illuminate Al Qaeda’s errors whenever possible.

A primary focus has become cyberspace, which is the global safe haven of terrorist networks. To counter efforts by terrorists to plot attacks, raise money and recruit new members on the Internet, the government has mounted a secret campaign to plant bogus e-mail messages and Web site postings, with the intent to sow confusion, dissent and distrust among militant organizations, officials confirm.

At the same time, American diplomats are quietly working behind the scenes with Middle Eastern partners to amplify the speeches and writings of prominent Islamic clerics who are renouncing terrorist violence.

At the local level, the authorities are experimenting with new ways to keep potential terrorists off guard.

In New York City, as many as 100 police officers in squad cars from every precinct converge twice daily at randomly selected times and at randomly selected sites, like Times Square or the financial district, to rehearse their response to a terrorist attack. City police officials say the operations are believed to be a crucial tactic to keep extremists guessing as to when and where a large police presence may materialize at any hour. “What we’ve developed since 9/11, in six or seven years, is a better understanding of the support that is necessary for terrorists, the network which provides that support, whether it’s financial or material or expertise,” said Michael E. Leiter, acting director of the National Counterterrorism Center.

“We’ve now begun to develop more sophisticated thoughts about deterrence looking at each one of those individually,” Mr. Leiter said in an interview. “Terrorists don’t operate in a vacuum.”

In some ways, government officials acknowledge, the effort represents a second-best solution. Their preferred way to combat terrorism remains to capture or kill extremists, and the new emphasis on deterrence in some ways amounts to attaching a new label to old tools.

“There is one key question that no one can answer: How much disruption does it take to give you the effect of deterrence?” said Michael Levi, a fellow at the Council on Foreign Relations and the author of a new book, “On Nuclear Terrorism.”

The emerging belief that terrorists may be subject to a new form of deterrence is reflected in two of the nation’s central strategy documents.

The 2002 National Security Strategy, signed by the president one year after the Sept. 11 attacks, stated flatly that “traditional concepts of deterrence will not work against a terrorist enemy whose avowed tactics are wanton destruction and the targeting of innocents.”

Four years later, however, the National Strategy for Combating Terrorism concluded: “A new deterrence calculus combines the need to deter terrorists and supporters from contemplating a W.M.D. attack and, failing that, to dissuade them from actually conducting an attack.”

For obvious reasons, it is harder to deter terrorists than it was to deter a Soviet attack.

Terrorists hold no obvious targets for American retaliation as Soviet cities, factories, military bases and silos were under the cold-war deterrence doctrine. And it is far harder to pinpoint the location of a terrorist group’s leaders than it was to identify the Kremlin offices of the Politburo bosses, making it all but impossible to deter attacks by credibly threatening a retaliatory attack.

But over the six and a half years since the Sept. 11 attacks, many terrorist leaders, including Osama bin Laden and his deputy, Ayman al-Zawahri, have successfully evaded capture, and American officials say they now recognize that threats to kill terrorist leaders may never be enough to keep America safe.

So American officials have spent the last several years trying to identify other types of “territory” that extremists hold dear, and they say they believe that one important aspect may be the terrorists’ reputation and credibility with Muslims.

Under this theory, if the seeds of doubt can be planted in the mind of Al Qaeda’s strategic leadership that an attack would be viewed as a shameful murder of innocents — or, even more effectively, that it would be an embarrassing failure — then the order may not be given, according to this new analysis.

Senior officials acknowledge that it is difficult to prove what role these new tactics and strategies have played in thwarting plots or deterring Al Qaeda from attacking. Senior officials say there have been several successes using the new approaches, but many involve highly classified technical programs, including the cyberoperations, that they declined to detail.

They did point to some older and now publicized examples that suggest that their efforts are moving in the right direction.

George J. Tenet, the former director of the Central Intelligence Agency, wrote in his autobiography that the authorities were concerned that Qaeda operatives had made plans in 2003 to attack the New York City subway using cyanide devices.

Mr. Zawahri reportedly called off the plot because he feared that it “was not sufficiently inspiring to serve Al Qaeda’s ambitions,” and would be viewed as a pale, even humiliating, follow-up to the 9/11 attacks.

And in 2002, Iyman Faris, a naturalized American citizen from Kashmir, began casing the Brooklyn Bridge to plan an attack and communicated with Qaeda leaders in Pakistan via coded messages about using a blowtorch to sever the suspension cables.

But by early 2003, Mr. Faris sent a message to his confederates saying that “the weather is too hot.” American officials said that meant Mr. Faris feared that the plot was unlikely to succeed — apparently because of increased security.

“We made a very visible presence there and that may have contributed to it,” said Paul J. Browne, the New York City Police Department’s chief spokesman. “Deterrence is part and parcel of our entire effort.”

Terrorists hold little or no terrain, except on the Web. “Al Qaeda and other terrorists’ center of gravity lies in the information domain, and it is there that we must engage it,” said Dell L. Dailey, the State Department’s counterterrorism chief.

Some of the government’s most secretive counterterrorism efforts involve disrupting terrorists’ cyberoperations. In Iraq, Afghanistan and Pakistan, specially trained teams have recovered computer hard drives used by terrorists and are turning the terrorists’ tools against them.

“If you can learn something about whatever is on those hard drives, whatever that information might be, you could instill doubt on their part by just countermessaging whatever it is they said they wanted to do or planned to do,” said Brig. Gen. Mark O. Schissler, director of cyberoperations for the Air Force and a former deputy director of the antiterrorism office for the Joint Chiefs of Staff.

Since terrorists feel safe using the Internet to spread ideology and gather recruits, General Schissler added, “you may be able to interfere with some of that, interrupt some of that.”

“You can also post messages to the opposite of that,” he added.

Other American efforts are aimed at discrediting Qaeda operations, including the decision to release seized videotapes showing members of Al Qaeda in Mesopotamia, a largely Iraqi group with some foreign leaders, training children to kidnap and kill, as well as a lengthy letter said to have been written by another terrorist leader that describes the organization as weak and plagued by poor morale.

Even as security and intelligence forces seek to disrupt terrorist operations, counterterrorism specialists are examining ways to dissuade insurgents from even considering an attack with unconventional weapons. They are looking at aspects of the militants’ culture, families or religion, to undermine the rhetoric of terrorist leaders.

For example, the government is seeking ways to amplify the voices of respected religious leaders who warn that suicide bombers will not enjoy the heavenly delights promised by terrorist literature, and that their families will be dishonored by such attacks. Those efforts are aimed at undermining a terrorist’s will.

“I’ve got to figure out what does dissuade you,” said Lt. Gen. John F. Sattler, the Joint Chiefs’ director of strategic plans and policy. “What is your center of gravity that we can go at? The goal you set won’t be achieved, or you will be discredited and lose face with the rest of the Muslim world or radical extremism that you signed up for.”

Efforts are also under way to persuade Muslims not to support terrorists. It is a delicate campaign that American officials are trying to promote and amplify — but without leaving telltale American fingerprints that could undermine the effort in the Muslim world. Senior Bush administration officials point to several promising developments.

Saudi Arabia’s top cleric, Grand Mufti Sheik Abdul Aziz al-Asheik, gave a speech last October warning Saudis not to join unauthorized jihadist activities, a statement directed mainly at those considering going to Iraq to fight the American-led forces.

And Abdul-Aziz el-Sherif, a top leader of the armed Egyptian movement Islamic Jihad and a longtime associate of Mr. Zawahri, the second-ranking Qaeda official, has just completed a book that renounces violent jihad on legal and religious grounds.

Such dissents are serving to widen rifts between Qaeda leaders and some former loyal backers, Western and Middle Eastern diplomats say.

“Many terrorists value the perception of popular or theological legitimacy for their actions,” said Stephen J. Hadley, Mr. Bush’s national security adviser. “By encouraging debate about the moral legitimacy of using weapons of mass destruction, we can try to affect the strategic calculus of the terrorists.”

As the top Pentagon policy maker for special operations, Michael G. Vickers creates strategies for combating terrorism with specialized military forces, as well as for countering the proliferation of nuclear, biological or chemical weapons.

Much of his planning is old school: how should the military’s most elite combat teams capture and kill terrorists? But with each passing day, more of his time is spent in the new world of terrorist deterrence theory, trying to figure out how to prevent attacks by persuading terrorist support networks — those who enable terrorists to operate — to refuse any kind of assistance to stateless agents of extremism.

“Obviously, hard-core terrorists will be the hardest to deter,” Mr. Vickers said. “But if we can deter the support network — recruiters, financial supporters, local security providers and states who provide sanctuary — then we can start achieving a deterrent effect on the whole terrorist network and constrain terrorists’ ability to operate.

“We have not deterred terrorists from their intention to do us great harm,” Mr. Vickers said, “but by constraining their means and taking away various tools, we approach the overall deterrent effect we want.”

Much effort is being spent on perfecting technical systems that can identify the source of unconventional weapons or their components regardless of where they are found — and letting nations around the world know the United States has this ability.

President Bush has declared that the United States will hold “fully accountable” any nation that shares nuclear weapons with another state or terrorists.

Rear Adm. William P. Loeffler, deputy director of the Center for Combating Weapons of Mass Destruction at the military’s Strategic Command, said Mr. Bush’s declaration meant that those who might supply arms or components to terrorists were just as accountable as those who ordered and carried out an attack.

It is, the admiral said, a system of “attribution as deterrence.”

Copyright 2008 The New York Times Company