"American and European investigators are interested in what they describe as Iran's purchase of nuclear centrifuge designs from Pakistan 16 years ago, largely to force the Pakistani government to face up to a pattern of clandestine sales by its nuclear engineers and to investigate much more recent transfers," including ones to North Korea in the late 1990s, the Times said.
The investigation has led Pakistani officials to question three senior nuclear scientists, the Times said. US intelligence experts have been permitted to assist in the questioning, according to The Washington Post.
Pakistan's suspected role in providing centrifuge designs to Iran was first reported Sunday in the Post, which said the blueprints provided a "tremendous boost" to Iran's pursuit of nuclear weapons.
Centrifuges can be used to enrich uranium, or spin it at supersonic speeds to produce a concentrated material used to make nuclear weapons.
When IAEA inspectors discovered working centrifuges at Iran's Natanz plant in February, Iran at first claimed to have built them alone.
But it later admitted they included second-hand components from another country, when inspectors detected traces of highly enriched uranium on the machines, which Iran insists it has never made.
Inspectors studying the machines also found that "the design is one of several known to have been stolen in the 1970s by a Pakistani nuclear scientist, Abdul Qadeer Khan, who later become known as the father of the Pakistani bomb," the Post said.
The centrifuge blueprints included design modifications similar to ones made by Pakistan, and the uranium traces were consistent with material produced in Pakistan, the daily said.
According to the Times, "a British expert who accompanied IAEA inspectors into Iran earlier this year identified the Iranian centrifuges as being identical to the early models that the Khan laboratories had modified from European designs."
The investigation has led Pakistan officials to back away from denials that Khan's laboratory was a source of technology for countries aspiring to gain nuclear weapons, the Times said.
All three of the scientists questioned -- Farooq Muhammad, Yasin Chohan and Sayeed Ahmad -- were close aides to Khan, who retired in 2001. Muhammad was in charge of dealing with foreign suppliers at his laboratory.
Pakistani media reports said Muhammad and Chohan were taken from their homes earlier this month, though Pakistani officials denied they had been arrested or detained. Chohan has since returned home but Muhammad is still being questioned.
Khan himself is the main focus of the investigation, but the government of Pakistani President Pervez Musharraf has been reluctant to challenge him because of his status as a national hero, the Times said.