Senior government and military officials on Friday said the needle of suspicion was increasingly being pointed at Abdul Qadeer Khan as more incriminating evidence came up against the top scientist.
"There is sufficient evidence of linkages between a handful of individuals in Pakistan and the international black market mafia trading in nuclear technology, with Khan being a primary figure in the shadowy business," one government official said on condition of anonymity.
Evidence collected so far strongly points to Khan's involvement in the alleged leaks in the late 1980s and early 1990s.
Pakistan has been probing a clandestine nuclear proliferation to Libya and Iran, ever since the United Nation's nuclear watchdog complained of it in a letter some months ago.
Probe teams have visited several countries including Libya.
Several nuclear scientists have been questioned and some have been detained.
Khan was questioned but not detained because of his public stature.
Pakistani President Pervez Musharraf has promised stern action against those responsible.
In his address at the World Economic Forum in Davos recently, Musharraf said those guilty of leaking nuclear technology would be treated as traitors.