Iran claims the work is related to road construction, while others, including the Institute for Science and International Security, suggest it's a cleanup operation before International Atomic Energy Agency (IAEA) inspectors arrive.

As lawmakers on Capitol Hill debate the Iranian deal, they must decide if Iran can be trusted.

Another question: Can U.S. intelligence services find out how close Iran is to building a nuclear bomb. If history is any judge, their record since World War II is not promising.

"Beginning all the way back in the 1940s with the Soviet Union at that time, the Communist Chinese, later on the Pakistanis, the Indians, the South Africans all developed programs … without the Western services, any of them apparently, including the IAEA once it had come into existence, knowing about them," Clare Lopez, with the Center for Security Policy, told CBN News.

Lopez believes Iran practices a dual nuclear strategy: one hidden and one for the world to see.  

"The entire program has always had a clandestine pathway to the bomb to it. I don't think we're anywhere near 10 to 15 years," she explained. "But the real program moved underground and remains to this day, in large part I think, a covert or clandestine nuclear weapons program and the proof of that is that every so often another site will be revealed."

Lopez agrees with Israel that the current nuclear deal fails to address Iran's secrets.

"I think that Prime Minister [Benjamin] Netanyahu is exactly right to say this deal, if it goes through, legitimizes the overt pathway to a bomb for Iran," she said. "But it doesn't even address the covert pathway to a bomb which continues unaffected by sanctions or anything else."

Given past failures of Western intelligence to uncover hidden nuclear programs around the world, U.S. lawmakers will have to determine they won't be fooled again.