Read the AP article linked above. Here's an excerpt:
Christopher Hill, the main U.S. envoy on the North Korea nuclear standoff, told The Associated Press that even though Libya got the nuclear material from a Pakistani black market nuclear network, the North Koreans must have known where their material would end up.
Hill, U.S. ambassador to South Korea who leaves next week to become assistant secretary of state for East Asian and Pacific affairs, said there is "physical evidence that the material that arrived in Libya had started its journey" in North Korea. He said the evidence was "beyond my reasonable doubt."
The "Pakistani black market" was the A.Q. Khan connection, and Khan had connections inside No. Korea.