Nuclear inspectors are expected to visit a site in the Iranian capital, Tehran, following evidence from satellite photographs that it was scraped clean earlier in 2004.
The Lavizan Shiyan Technical Research Centre, in a north-eastern suburb of the city, has been under mounting suspicion of harbouring secret military activities since it was named by an Iranian opposition group in 2003.
Now two commercial satellite images, the first on 11 August 2003 and the second on 22 March 2004, show that the site's buildings have been razed, its features obliterated and its ground cleared.
"The images show that Iran has taken dramatic steps that make it difficult to discover what was happening there," says Corey Hinderstein from the Institute for Science and International Security in Washington DC. It was the institute which found and released the photographs.
Independent nuclear experts regard the satellite photographs of Lavizan Shiyan as important new evidence. "Iran is clearly trying to hide something - and there is suggestive, though not conclusive, evidence that the something is nuclear," says Matt Bunn, a nuclear policy adviser to former US President Bill Clinton, now at Harvard University. "Iran clearly owes the world an explanation."
John Pike, director of globalsecurity.org, a defence policy group based in Alexandria, Virginia, agrees: "The images suggest that there are important elements of Iran's nuclear program that have not been disclosed by Iran, and may not be reflected in the IAEA's current understanding of Iran's nuclear efforts."
The National Council for Resistance of Iran claimed in May 2003 that Lavizan Shiyan was a research facility for biological weapons. But since then investigations by the US government, the UN's International Atomic Energy Agency (IAEA) and other countries have uncovered evidence that the site had imported a radiation monitor and bought spare parts for it.
This does not prove that nuclear weapons were being developed on the site, Hinderstein points out. But the monitor was "out of place" at a location which Iran had not declared as having any nuclear activities.
The IAEA told New Scientist that it had only just seen the satellite images. "We have asked for clarification from Iran and will visit if we deem it important," said an agency spokeswoman. "At the moment, we do not know whether it is nuclear related."
Hossein Mousavian, Iran's chief delegate to the IAEA in Vienna, told reporters that inspectors would be able to visit the site. "There is nothing there," he said. If it has been thoroughly cleaned up, inspectors may have difficulty detecting residual radioactivity or chemicals.
Iran has consistently denied that is developing nuclear weapons, but insisted on its right under the Nuclear Non-Proliferation Treaty to expand its civil nuclear power programme. One of the technologies it has been developing has been uranium enrichment, which can be used to make fuel for power stations or bombs.
At a meeting in Vienna last week, the IAEA's 35 governors approved a resolution deploring Iran's lack of "full, timely and proactive" co-operation in disclosing its nuclear activities. In response Hasan Rowhani, head of the country's Supreme National Security Council, warned that it might restart uranium enrichment, suspended in October 2003 at the request of the UK, France and Germany.