What they do not have yet is a reliable weapon than can be delivered by missile. They also lack supplies of atomic weapons materials to build out their arsenal.
Panetta certainly knows whether this is true or not.
His public remarks are not connected with the real situation, nor is he foolish enough to believe what he is saying. He is saying what he is told to say.
Iran long ago decided to build nuclear weapons. That much is obvious just by looking at the missile infrastructure they are working on - These delivery systems are expensive and very hard to master so building such missiles only makes sense in the context of nuclear device tipped ICBMs.
Taken as a whole, Iran's military infrastructure development is geared to fielding a nuclear arsenal and the necessary delivery systems. It takes a lot more components than just a bomb to make a credible nuclear arsenal and Iran is pushing forward on all fronts to develop all of the necessary components of a system that can be used as an offensive nuclear first strike capability.
Iran probably has had one or more nuclear explosive devices of one form or another for some time - they just don't have the proper delivery systems to attack with them, and more importantly, they don't yet have enough bombs and delivery systems to discourage a retaliatory strike after they have made an attack.
For example, lets say Iran attacked Israel with a nuclear ICBM. This could provoke a response by the US.
However, if Iran detonated a nuclear device within the US and then announces that they had infiltrated 20 such weapons into the US across the US-Mexico border and hidden them in American cities to destroy those cities if the US did not give into Iran's demands there could be a big problem for the US and would probably keep us from launching a retaliatory strike against Iran.
FWIW, Obama Administration border control policies seem tailor made to facilitate such nuclear bombs being smuggled across the border by Iranian Hezbollah forces know to be working openly in Mexico