"Our inspectors went yesterday to Lavizan (the suspect site). The Iranians said it was a former R and D military site," International Atomic Energy Agency (IAEA) director general ElBaradei told reporters in Novo Ogaryovo outside Moscow, where he met with Russian President Vladimir Putin.
The Iranians said the site "was used as a physics institute and later on for biotechnology R and D ... for medicine," ElBaradei said.
Suspicion has surrounded the site since satellite images from a US commercial firm showed that buildings which had been there in August had been razed to the ground by March and that topsoil had been taken away.
The Washington think tank the Institute for Science and International Security (ISIS) said on its website that this set alarm bells ringing "because it is the type of measure Iran would need to take if it was trying to defeat the powerful environmental sampling capabilities of IAEA inspectors."
Environmental sampling involves swipes taken to find traces of radiation.
"I have to wonder if it is the whole story, particularly since they took down all the buildings and razed the site," ISIS scientist David Albright told AFP Tuesday, referring to Iran's claim there was no weapons work in Lavizan.
"Their declaration is rather vague and looks like it covers all the bases. Medical research has to be mentioned" to explain whole body count machines found at the site, machines which measure radiation contamination, he said.
The United States claims that Iran is hiding an atomic weapons program and has urged the IAEA to bring Tehran before the UN Security Council for possible sanctions.
ElBaradei said Lavizan had radiation body counters, which measure radiation contamination in humans, and which could have been US-made.
He said the IAEA has not determined if the site was for weapons development or peaceful purposes, as Iran claims.
IAEA inspectors have taken environmental samples at Lavizan. "We are continuing to clarify that this site was not nuclear related. We are pleased with the Iranian cooperation," ElBaradei said.
Albright said the next step will be for the IAEA to get the results of the environmental samples, which can take four to six weeks, and to see if the whole body count machines have all the original parts as shipped from the United States.
"These machines can be used for all kinds of (peaceful) purposes. You have it in a hospital if you are giving radiation treatments... The IAEA will look however to see if the whole body counters are modified" to look for (bomb-grade) plutonium for example, or if the original parts have been taken out and put back in, which would also be suspicious, Albright said.
"If Iran is innocent, why did they tear the site down," Albright said. "Things look really bad when people destroy things unilaterally," he added, noting that the IAEA was not notified of the site being razed.
The Lavizan site was brought to public attention in May last year, when an Iranian opposition group, the National Council of the Resistance of Iran, alleged that it was home to a biological weapons research facility.
Iran has remained under heavy international suspicion. The 35-nation board of the Vienna-based IAEA passed a resolution on June 18 rebuking Tehran for failing to come clean about its nuclear program.
Iran confirmed Sunday it had razed the Lavizan site, but insisted it was to make a park and not to cover up nuclear weapons activities as the United States has alleged.
On June 17, the United States accused Iran of razing nuclear sites to hide banned nuclear activity.
"It's deplorable but not surprising that Iran's deception has gone to the extent of bulldozing entire sites to prevent the IAEA from discovering evidence of its nuclear weapons program," US State Department spokesman Richard Boucher said.