By Dafna Linzer
Washington Post Staff Writer
Tuesday, August 23, 2005; A01

Traces of bomb-grade uranium found two years ago in Iran came from contaminated Pakistani equipment and are not evidence of a clandestine nuclear weapons program, a group of U.S. government experts and other international scientists has determined.

"The biggest smoking gun that everyone was waving is now eliminated with these conclusions," said a senior official who discussed the still-confidential findings on the condition of anonymity.

Scientists from the United States, France, Japan, Britain and Russia met in secret during the past nine months to pore over data collected by inspectors from the International Atomic Energy Agency, according to U.S. and foreign officials. Recently, the group, whose existence had not been previously reported, definitively matched samples of the highly enriched uranium -- a key ingredient for a nuclear weapon -- with centrifuge equipment turned over by the government of Pakistan.

Iran has long contended that the uranium traces were the result of contaminated equipment bought years ago from Pakistan. But the Bush administration had pointed to the material as evidence that Iran was making bomb-grade ingredients.

The conclusions will be shared with IAEA board members in a report due out the first week in September, according to U.S. and European officials who agreed to discuss details of the investigation on the condition of anonymity. The report "will say the contamination issue is resolved," a Western diplomat said.

U.S. officials have privately acknowledged for months that they were losing confidence that the uranium traces would turn out to be evidence of a nuclear weapons program. A recent U.S. intelligence estimate found that Iran is further away from making bomb-grade uranium than previously thought, according to U.S. officials.

The IAEA findings come as European efforts to negotiate with Iran on the future of its nuclear program have faltered, and could complicate a renewed push by the Bush administration to increase international pressure on Tehran.

U.S. officials, eager to move the Iran issue to the U.N. Security Council -- which has the authority to impose sanctions -- have begun a new round of briefings for allies designed to convince them that Iran's real intention is to use its energy program as a cover for bomb building. The briefings will focus on the White House's belief that a country with as much oil as Iran would not need an energy program on the scale it is planning, according to two officials.

France, Britain and Germany have been trying for two years to convince Iran that it could avoid Security Council action if it gives up sensitive aspects of its nuclear energy program that could be diverted for weapons work. Iran has said it has no intention of making nuclear weapons and will not give up its right to nuclear energy. Iran has offered to put the entire program under IAEA monitoring as a way of alleviating international concerns. But European and U.S. officials have rejected that offer because it would still allow Iran access to bomb-making capabilities.

Iran built its nuclear program in secret over 18 years with the help of Abdul Qadeer Khan, a top Pakistani official and nuclear scientist who sold spare parts from his country's own weapons program to Iran, Libya and North Korea. Khan's black-market dealings were uncovered in 2003. He confessed on national television, was swiftly pardoned by Pakistan's president, Gen. Pervez Musharraf, and is now under house arrest.

Pakistan has denied IAEA inspectors access to Khan and to the country's nuclear facilities, but earlier this year it agreed to share data and some equipment with the inspectors to expedite the Iran investigation. Among the equipment were discarded centrifuge parts that match those Khan sold to Iran.

John R. Bolton, now U.S. ambassador to the United Nations, served as the administration's point man on nuclear issues during President Bush's first term. He suggested during congressional testimony in June 2004 that the Iranians were lying about the contamination.

"Another unmistakable indicator of Iran's intentions is the pattern of repeatedly lying to and providing false and incomplete reports to the IAEA," Bolton said. "For example, Iran first denied it had enriched any uranium. Then it said it had not enriched uranium more than 1.2 percent. Later, when evidence of uranium enriched to 36 percent was found, it attributed this to contamination from imported centrifuge parts."

The IAEA, in its third year of an investigation in Iran, has not found proof of a weapons program. But a few serious questions, some connected to Iran's involvement with Khan, remain unanswered. While the investigation has been underway, Iran and the three European countries have been trying to reach a diplomatic accommodation. Their negotiations fell apart this month and Iran resumed some nuclear work it put on hold during the talks.

In the meantime, European officials convened an IAEA board meeting two weeks ago to discuss Iran's actions and sought a new report for this week on its program. But the report was pushed back to Sept. 3 so that the group of scientists, including officials from the Energy Department, could meet one last time to draft an account of its findings, according to U.S. and European officials.

The IAEA had put together the group of experts in an effort to foster cooperation but also to eliminate the possibility that its findings would be challenged by the White House, officials said. In the run-up to the Iraq invasion in March 2003, the White House rejected IAEA findings that cast doubt on U.S. assertions about then-Iraqi President Saddam Hussein's arsenal. The IAEA findings turned out to be correct, and no weapons of mass destruction have been found in Iraq.