ASHINGTON, Sept. 29 — The Central Intelligence Agency secretly began to send teams of American officers to northern Afghanistan about three years ago in an attempt to persuade the leader of the anti- Taliban Afghan opposition to capture and perhaps kill Osama bin Laden, according to American intelligence officials.

The covert effort, which has not been previously disclosed, was based on an attempt to work with Ahmed Shah Massoud, who was then the military leader of the largest anti- Taliban group in the northern mountains of Afghanistan, and to have his forces go after Mr. bin Laden. Mr. Massoud was himself fatally wounded only two days before the attacks on the Pentagon and the World Trade Center, and the C.I.A. believes that he was assassinated by members of Mr. bin Laden's organization.

The C.I.A.'s clandestine efforts to deal with Mr. Massoud were among the most sensitive and highly classified elements of a broader long-term campaign, continuing unsuccessfully through the end of the Clinton administration and into the Bush administration, to destroy Mr. bin Laden's terrorist network. The American campaign against Mr. bin Laden intensified after the August 1998 bombings of two United States Embassies in East Africa, which transformed the Saudi-born exile into America's most wanted terrorist.

Today, the hunt for Osama bin Laden and his lieutenants in Al Qaeda, the terrorist network he leads from his sanctuary in Afghanistan, has escalated to wartime levels. The Bush administration is considering a full range of overt and covert military and intelligence proposals that Washington policy makers would have considered too risky or unworkable before the attacks on the World Trade Center and the Pentagon.

But according to current and former intelligence officials and other policy makers, the United States has been trying to kill bin Laden and destroy Al Qaeda for years, as the terrorist organization has become more ruthless and ambitious in its efforts to attack American interests around the world.

Clinton administration lawyers determined that the United States could legitimately seek to kill Mr. bin Laden and his lieutenants despite the presidential ban on assassinations, according to current and former American officials. The lawyers concluded that efforts to hunt and kill Mr. bin Laden were defensible either as acts of war or as national self defense, legitimate under both American and international law. As a result, President Clinton did not waive the executive order banning assassinations.

There have been an array of unsuccessful attempts to target Mr. bin Laden and disrupt or destroy Al Qaeda, American officials say. The Clinton administration even considered mounting a secret effort to steal millions of dollars from the bin Laden terrorist network by siphoning it out of the international financial system, but discarded the scheme because of objections from the United States Treasury about the implications for world finance.

The United States launched cruise missiles against a meeting Mr. bin Laden was believed to be attending, encouraged Mr. Massoud and other Afghan leaders to try to capture him, and received a secret report from one Afghan group last year about its failed attempt to assassinate Mr. bin Laden.

The United States also led an international effort to shut down Afghanistan's airline, which American intelligence officials believed was being used by Al Qaeda to ship money and personnel around the world, while also pressuring other nations to arrest and disrupt Al Qaeda cells.

"This was a top priority for us over the past several years, and not a day went by when we didn't press as hard as we could," said Samuel R. Berger, national security adviser in the Clinton administration. "But this is a tough, tough problem. I think we were pushing it as hard as we could. And I think the Bush administration is handling it in a smart way."

But until the devastating attacks on New York and Washington, the American-led efforts to hunt Mr. bin Laden lacked the sense of urgency that prevails today. American intelligence and law enforcement officials grew complacent about the threat of a domestic attack by Al Qaeda, failed by their own admission to share information adequately or coordinate their efforts, and were caught by surprise on Sept. 11.

Washington did not build a strong international coalition to focus on defeating Al Qaeda, which was seen by other nations largely as an American problem. Banks in Europe and the Middle East repeatedly balked at American pressure to cut off Al Qaeda financing, while wealthy individuals in Persian Gulf states — sometimes in the guise of donating to Islamic charities — continued to provide financial support to Al Qaeda.

At the same time, Al Qaeda was rapidly evolving into a larger and more complex terrorist threat, making it difficult for the United States to keep up with its scope and abilities. Mr. bin Laden's great achievement within the terrorist world has been to forge alliances with other Islamic extremist groups under the umbrella of Al Qaeda, providing them financing, training and a sanctuary in Afghanistan, while encouraging coordinated action.

The United States had only a hazy understanding of Mr. bin Laden's growing significance before 1996, when an Al Qaeda insider, Jamal Ahmed Al-Fadl, defected to the United States and began to describe the extent of Mr. bin Laden's plans and objectives. Based largely on Mr. al- Fadl's information, a federal grand jury indicted Mr. bin Laden on terrorist conspiracy charges in June 1998, just two months before the twin bombings of the American Embassies in Kenya and Tanzania.

The embassy bombings forced Washington to recognize that Mr. bin Laden had become a major national security threat. Sometime after the bombings, the C.I.A. began its efforts to work with Mr. Massoud against Mr. bin Laden, American officials said.

The officials declined to provide many details of the effort. But officials say that C.I.A. officers secretly traveled to Mr. Massoud's mountain stronghold in northern Afghanistan and opened talks in an effort to fashion an anti-bin Laden alliance.