NEW YORK March 21: Investigators from the United States and other nations now believe that the black market network run by Pakistani scientist A. Q. Khan was selling not only technology for enriching nuclear fuel and blueprints for nuclear weapons, but also some of the darkest of the bomb makers’ arts: the hard-to-master engineering secrets needed to fabricate nuclear warheads, says the New York Times. In a report on Monday the Times said that the investigators’ suspicions were initially raised by the discovery of step-by-step instructions, some of which appear to have come from China and Pakistan, among the documents recovered last year from Libya. More recently, investigators have found that the Khan network had offered similar materials to Iran.

The inability of intelligence officials to track down the whereabouts of the bomb-making instructions underscores the fact that more than a year since Mr Khan’s arrest and pardon by President Gen Pervez Musharraf, there are still many mysteries about what exactly the Khan network was selling, and to whom, the paper said.

The United States has not been allowed to interview Dr Khan, and US Secretary of State Condoleezza Rice raised concerns about cooperation in the nuclear investigation when she met Gen Musharraf last week, the paper noted.

But American officials and the International Atomic Energy Agency are beginning to extract information from Dr Khan’s chief deputy Buhari Sayed Abu Tahir, who is in jail in Malaysia. “It’s becoming clearer to us that Khan was selling a complete package,” said a senior American official involved in the setting of nuclear strategy. “Not a turnkey operation — that would be overstating it — but close to it.”

The secrets range from how to cast uranium metal into the form needed at the core of a bomb to how to build the explosive lenses that compress the core and start the detonation, the paper said.

The discoveries have set off a debate in the intelligence community about whether those technological skills made their way to North Korea and Iran. President Bush has vowed he would not tolerate either country obtaining a nuclear weapon.

Iran was a customer of the Khan network, and while it appears to have turned down the offer of the engineering secrets in 1987, some intelligence officials are concerned that it picked up the technology elsewhere. North Korea, which is believed to have two separate bomb projects under way, also did business with the Khan network, although precisely what it obtained is not clear.

European and American officials said they considered the 1987 transaction some of the best evidence that Iran sought, starting at least 18 years ago, to assemble the technologies needed to build a nuclear arsenal.

“It adds a piece to the puzzle that makes the whole thing more incriminating,” a European official said. “But is this a smoking gun? No. Does this make people more suspicious? Yes.”

The weeks leading up to Ms Rice’s visit to China this weekend, American officials provided their Chinese counterparts with a stream of new information about North Korea’s nuclear program, but it is not clear how much detail they went into about their latest suspicions. The Chinese, for their part, are sceptical of the quality of the American intelligence, the paper said.

To investigators and other experts, the discovery that Dr Khan was selling step-by-step directions for making crucial parts of a bomb was startling.

“The real secrets are in the details of the metallurgy, the manufacturing and the engineering,” said Siegfried S. Hecker, director of the Los Alamos weapons laboratory from 1986 to 1997 and now a senior fellow there.