They were given extraordinary access during two-week-long visits in October and December with the help and encouragement of Libyan leader Colonel Moammar Khadafy who met personally with them, senior intelligence officials said.
The secret dealings culminated Friday with the stunning annoucement in London and Washington and Tripoli that Libya has agreed to fully disclose and dismantle its programs to build unconventional weapons under international supervision.
"It wasn't individual things that we were shown that we were blown away by," said one senior intelligence oficial. "What seems very surprising is the extent we were given access to this stuff over the course of the visits."
They visited nine nuclear-related sites, including a Soviet-built research reactor at Tajura, and dozens of other locations across the country related to chemical, biological, and missile programs.
They were allowed to take samples and photographs and interview scientists, said two senior intelligence officials who briefed reporters on condition of anonymity.
"The analysts that I talked to that were on the team were quite excited about the level of access that they eventually got," the official said.
The officials would not disclose the specific sites visited, what they found in them, or what they learned about Libya's suppliers.
But they said they hoped intelligence gathered would help disrupt or interdict networks supplying weapons programs of other countries or of terrorist organizations.
"I would say we'll benefit significantly from being able to unravel this particular program and use it as a template for collections against other programs, as well as stop other programs before they get well started," said a the second senior intelligence official.
They said that what they found vindicted what the US intelligence community already knew about Libya's unconventional weapons program.
"The Libyans were substantially farther along in the uranium enrichment program than had been publicly disclosed before," the first official said.
"By far the most significant revelation they made was their disclosure of centrifuges. That was quite a milestone in their admissions," he said
However, they saw no functioning cascade of centrifuges required to produce fissile materials for nuclear weapons, and the Libyans denied having actually enriched uranium, the official said.
On other weapons program, they were shown "tens of tonnes" of sulphur mustard agent produced at Libya's Rabta chemical plant about a decade ago, as well as hundreds of 250 kilogram aerial bombs equipped to deliver mustard agent.
They said the teams were also shown dual-use precursor chemicals that could be used to produce nerve agent, but would not say whether any active chemical warfare production lines were found.
"Those are details that will come that from the Libyans or further inspections," the first intelligence official said.
They were given access to medical and pharmaceutical facilities that could also be used for production of biological warfare agents, as well to scientists.
"But (what) we did not find in the limited time we had to sample and interview was direct evidence of biological weapons programs," one of the officials said. "The Libyans did not acknowledge having biological weapons."
They also were shown North Korean-supplied Scud C missiles with a range of about 800 kilometers, they said.
The series of secret meeting began in mid-March. But the biggest "breakthrough" came between the two visits by technical experts in October and December after the Libyans learned how detailed US knowlege of their programs was, the officials said.
"It pretty quickly convinced them that they needed to be even more forthcoming than they might have been otherwise," one of the officials said.
The second official, who led the US teams that went to Libya, described Khadafy as the "driver" of the process and said he appeared to have a variety of motives, including concerns about his legacy, as well as challenges from "extremism" within Libya.
The official raised the possibility of working more closely with the Libyans in the future against extremists.
"During the meetings with Colonel Khadafy, he was consistent throughout in his intentions to stay with the admission and elimination of their WMD programs," the official said.