Islamabad has questioned Abd al-Qadir Khan, revered as a national hero for developing the country's nuclear bomb, and several colleagues in recent weeks.
The probe came after a UN nuclear agency began investigating possible links between the Pakistani and Iranian nuclear programmes.
Information Minister Shaikh Ahmad Rashid said eight people, including three military officers and three scientists, were among those being "debriefed" on Sunday. He gave no further details.
A senior government official said Islam al-Haq, Khan's principal secretary and a former major, was detained for questioning on Saturday evening in Islamabad.
Pakistan says some of its scientists may have been driven by "personal ambition or greed" to export technology to Tehran but denies the government itself was ever involved in such technology transfer.
Islamabad is a staunch ally in Washington's so-called "war against terror".
Al-Haq's detention came hours after President Pervez Musharraf said Pakistan faced serious accusations of spreading "terrorism" and nuclear technology.
"We have to assure the world that we are a responsible nation and we will not allow proliferation of nuclear weapons," he told a noisy parliament session on Saturday.
President Pervez Musharraf wants
to send reassurances to world
The New York Times on Saturday quoted US law enforcement officials as saying they were looking into whether the Pakistani government was involved in a plot by a South African businessman to export trigger devices that could be used for nuclear arms.
Al-Haq was serving as a director at the Khan Research Laboratories (KRL), the country's top uranium enrichment lab, set up by Khan in the 1970s near Islamabad.
Al-Haq's wife, Nilofar Islam, said she had no information about her husband's whereabouts.
Pakistani intelligence officials questioned at least three scientists working with KRL last month after diplomats in Vienna said the International Atomic Energy Agency was investigating a possible link between Islamabad and Tehran.
Iran has admitted using centrifuge designs that appear to be identical to those used in Islamabad's past quest for an atom bomb.