Satellite images going back to 2012 show earth-moving equipment and water tanks at the facility. Meanwhile, images from 2013 show large areas of ground that have been covered by asphalt.
Mr Fitzpatrick says: "Given the clean-up efforts that have apparently been underway since the IAEA in early 2012 sought to go back to Parchin, it is unlikely that the IAEA would be able to find any incriminating evidence there.
"It is therefore mysterious why Iran has steadfastly refused access - after having tentatively agreed in 2012 to allow it. "
Mr Fitzpatrick says the answer is probably related to "pride and internal politics", adding: "The hardliners do not want to grant any concessions unless Iran is suitably rewarded."