A senior Bush administration official said on Monday that Abd al-Qadir Khan, the Pakistani scientist, cut a "very lucrative" deal in the 1990s to supply Libya with almost everything it needed for its nuclear arms program, from centrifuges to on-site training.
Khan has admitted selling nuclear secrets to Iran and North Korea.
He was pardoned by Pakistani president Pervez Musharraf. But US administration and congressional figures say Khan may have sold his nuclear wares to other nations as well.
Tripoli agreed to give up its nuclear weapons programme in December, prompting the Bush administration to ease some US sanctions.
"The developing picture… indicates that the Khan network received at least $100 million for supplying technology, equipment and know-how" to the Libyans alone, said Jim Wilkinson, deputy White House national security advisor.
It was the White House's first public accounting of Khan's business dealings with Libya, and it topped previous estimates by diplomats of transactions totalling between $50 million and $100 million.
The Khan network stretched across Europe, Africa, the Middle East and Asia. Muhammad al-Baradai, director-general of the International Atomic Energy Agency, has described it as a "supermarket" for countries wanting the bomb.
While administration officials touted progress combating proliferation, they acknowledge there are other nuclear suppliers out there besides Khan.
Musharraf is crucial to US
operations in Afghanistan
"There are many other sources of supply for this type of (nuclear) equipment as well as in the chemical and biological areas," one official said.
Another official told reporters Libya obtained dual-purpose equipment from European firms. He declined to name them.
The Libyans first met with Khan in Istanbul in the late 1990s and shortly after began to work with the network to acquire the centrifuges and warhead designs, said a senior administration official.
His network would eventually become the "principle supplier for the entire (Libyan) programme," the official added.
"The (Libyan) programme was much more advanced than we had assessed," said Robert Joseph, a senior NSC official.
The United States has chosen not to criticise Musharraf's pardon of Khan, partly out of fear that a public trial could implicate many in the Pakistani elite and undermine Musharraf's rule in Pakistan.
Musharraf's support is crucial to US operations against al-Qaida and Taliban forces in neighbouring Afghanistan.