The German freighter, en route from Dubai, was intercepted and diverted to Taranto, southern Italy, shortly after it passed through the Suez Canal in October.
Intelligence agents boarded it and seized the cargo, which was not listed on its manifest Even though Tripoli had been engaged in secret talks about its weapons programme for six months before the seizure, the White House believes the interception may have been a factor in its declaration of full cooperation earlier this month.
With Iraq's weapons of mass destruction still undiscovered and Saddam Hussein captured only a week before, the gesture was hailed as a victory for the war on terror.
At the time Muammar Gadafy's government had not yet given a date for US and British intelligence officers to visit weapons-development sites. After the seizure, inspectors were in Libya within two weeks, according to US sources.
At the time, however, some believed the incident might have made it more difficult to win Libya over.
The interception and seizure were hailed as a victory for the new proliferation security initiative, an agreement between 11 countries to stop planes and ships suspected of carrying banned weapons or missile technology.
"It is clearly a success for the proliferation initiative, but it is also an allied success, especially for the Germans and Italians," a US government official said.
He said the German government and the shipping company had been "extremely cooperative".
This was the first seizure of its kind under the agreement.
The shipment, first reported in the Wall Street Journal earlier this week, shed light on Libya's arms programmes.
Last week UN investigators were shown dozens of centrifuges and other pieces of equipment in Libya, but found no evidence that it had enriched uranium.
The director of the International Atomic Energy Agency, Mohamed ElBaradei, said the equipment showed Libya was at an "early stage" of its weapons programme.
The centrifuges are believed to have been stolen several years ago from a European consortium, Urenco, and used to enrich uranium in several other countries.
"There is a black market in this material," a US official said.