US intelligence officials have told Congress that satellite imagery of the Parchin military base shows efforts to "sanitize" the facility in advance of an expected inspection by the IAEA sometime before October 15.

Parchin has long been suspected of conducting nuclear research and possibily tests for a nuclear trigger - an implosion device that would be used with a plutonium bomb.

Josh Rogin and Eli Lake:

The U.S. intelligence community has informed Congress of evidence that Iran was sanitizing its suspected nuclear military site at Parchin, in broad daylight, days after agreeing to a nuclear deal with world powers.

For senior lawmakers in both parties, the evidence calls into question Iran’s intention to fully account for the possible military dimensions of its current and past nuclear development. The International Atomic Energy Agency and Iran have a side agreement meant to resolve past suspicions about the Parchin site, and lawmakers' concerns about it has already become a flashpoint because they do not have access to its text.

Intelligence officials and lawmakers who have seen the new evidence, which is still classified, told us that satellite imagery picked up by U.S. government assets in mid- and late July showed that Iran had moved bulldozers and other heavy machinery to the Parchin site and that the U.S. intelligence community concluded with high confidence that the Iranian government was working to clean up the site ahead of planned inspections by the IAEA.

The intelligence community shared its findings with lawmakers and some Congressional staff late last week, four people who have seen the evidence told us. The Office of the Director of National Intelligence briefed lawmakers about the evidence Monday, three U.S. senators said.

“I am familiar with it,” Senate Intelligence Committee Chairman Richard Burr told us Tuesday. “I think it’s up to the administration to draw their conclusions. Hopefully this is something they will speak on, since it is in many ways verified by commercial imagery. And their actions seem to be against the grain of the agreement.”

Burr said Iran’s activities at Parchin complicate the work of the IAEA inspectors who are set to examine the site in the coming months. IAEA's director general, Yukiya Amano, was in Washington on Wednesday to brief lawmakers behind closed doors about the side agreements.

“They are certainly not going to see the site that existed. Whether that’s a site that can be determined what it did, only the technical experts can do that,” Burr said. “I think it’s a huge concern.”

Finding evidence of nuclear research would be difficult, but not impossible. But finding traces of the conventional explosives used in implosion tests are another story. Implosion uses shaped charges of conventional explosives and "lenses" to direct the energy uniformly around a sphere of plutonium about the size of a grapefruit. With other initiators, the resulting explosion squeezes the plutonium to the size of a tennis ball, thus causing the sphere to go critical and explode.

Iran currently has very little plutonium. But once they ramp up the the reactor at Bushehr that won't be a problem. Plutonium is a by product of the nuclear fuel cycle and even small amounts can get "lost."

Implosion tests at Parchin reportedly go back to as early as 2003, and may have continued through 2012. A building on the base was almost certainly used as an explosives chamber, although it may be impossible to say exactly what was being tested.

Not surprisingly, there is nothing in the agreement with Iran that prevents them from sanitizing nuclear sites. Just one more reason why this is such a horrible agreement.