Pakistan never cracked down on its scientists when former President Clinton and other U.S. officials shared their suspicions with Pakistani leaders, former U.S. officials say.

"The response was, 'Yes, we'll examine your concerns, but we don't believe they are well founded,'" said Robert Einhorn, who was the head arms control official in the State Department from 1999 to 2001.

While Islamabad and Washington squabbled about the evidence, the Khan network provided sophisticated technology to Libya, North Korea and Iran, three countries the United States considered among the most dangerous.

A decade earlier, the Reagan administration had looked the other way on Pakistan's nuclear program, said Stephen P. Cohen, a State Department expert on the region from 1985 to 1987. Back then, Washington used Pakistan as a conduit for sending weapons and money to guerrillas fighting the Soviet invasion of Afghanistan.

"They were covering up our involvement in Afghanistan, pretending we played no role in Afghanistan, so they expected us to cover up their role in procuring a weapons system they saw as vital to their survival," said Cohen, now with the Brookings Institution think tank.

American officials scolded Pakistan repeatedly for buying nuclear technology from sources in Europe, Asia and the United States, Cohen said. But often those warnings were with "a wink and a nod" that Washington would tolerate those activities, he said.

A declassified State Department memo from 1983 says Pakistan clearly had a nuclear weapons program that relied on stolen European technology and "energetic procurement activities in various countries."

Cohen said the United States suspected Pakistan was helping Iran in the late 1980s, in part because Pakistan had cooperated with Iran on nuclear matters before Iran's 1979 Islamic revolution. The evidence, however, was murky, Cohen said.