It was an ordinary number, just a dozen digits: 873682505331.

But it gave U.S. intelligence and law enforcement the key to the prosecution of four of Osama bin Laden's followers this year for their roles in the 1998 terrorist bombings of two American embassies in East Africa.

The number rang bin Laden's satellite telephone, a laptop-sized device that linked his hideout in the mountains of Afghanistan to a global network of followers - and to the National Security Agency at Fort Meade, which intercepted the plotters' calls.

While not explicit enough to allow U.S. authorities to move in time to prevent the bombings, the intercepts helped track the terrorists and send them to prison.

Since the East Africa bombings, however, the NSA has had far less success in picking up bin Laden's communications, according to people knowledgeable about U.S. intelligence.

Possibly put on guard by the embassy bombing investigation, the Saudi dissident and his terrorist lieutenants evidently have become far more cautious in their communications.

"They've been very careful and compartmented," said one former senior intelligence official.

James Bamford, author of two books on the NSA, was told by agency sources that in the late 1990s, NSA officers would sometimes play a tape of bin Laden talking with his mother to impress visitors holding high-level security clearances. But early this year, Bamford said, a source told him that the agency "had totally lost bin Laden's calls."

"They lost all track of him," Bamford said. "It could be that he uses couriers for really important communications. Or it could be he's using encryption."

If it is true that the NSA can no longer monitor bin Laden's communications - and the agency isn't saying - the loss might have played a role in the failure of U.S. intelligence to pick up warning of Tuesday's terrorist attacks. Officials have called bin Laden their prime suspect.

Now, the loss will hinder investigators as they try to hunt down those who conceived, planned and paid for the bloodiest terrorist assault in U.S. history.

It has already triggered a new debate over government control of encryption, posing the authorities' desire to intercept terrorists' communications against Americans' freedom to communicate without fear of government snooping.

Over the past decade, encryption - software or hardware used to scramble communications so that only the intended recipients can understand them - has become widely available and virtually unbreakable. And, as officials have long feared, encryption is becoming a deadly weapon in the terrorists' arsenal.

In the aftermath of Tuesday's attacks, Sen. Judd Gregg, a New Hampshire Republican, called for a global prohibition on encryption products that do not have a back door built in for government surveillance.

"This is something we need international cooperation on ... to get the information that allows us to anticipate and prevent what occurred in New York and Washington," Gregg told the Senate.

Unworkable and unwise



Cryptographer Phil Zimmermann - the creator of PGP, for Pretty Good Privacy, a powerful and widely used e-mail encryption program - said such a ban would be unworkable and unwise. He said PGP is used by human rights activists worldwide collecting information on government repression and even genocide. "If we make momentous political decisions under such incredible emotional pressure," he said, "we're bound to make terrible decisions."

Bin Laden and his allies may be using several methods to hide from the NSA. He may simply have dropped all electronic communications, insisting that sensitive messages be hand-carried. His associates in urban areas may make calls from random public telephones using untraceable, prepaid phone cards, and using vague language.