The Guard's aerospace chief, Gen. Ami Ali Hajizadeh, told state-run Press TV that that the RQ-170 Sentinel craft had not carried out missions over nuclear facilities before it went down in December 2011 near the eastern border with Afghanistan.
Tehran had previously said it recovered information from the top-secret stealth aircraft, but Monday's announcement suggests technicians may have broken encryptions.
"All data from the drone have been completely decoded. We know where it travelled step by step," Hajizadeh was quoted as saying. "After decoding, our experts discovered that this drone had not carried out even a single nuclear mission over Iran."
Hajizadeh said Iran had captured the drone and decoded its data without any assistance, including from its allies China and Russia. Iran has said it would reverse-engineer the drone and build its own version.
The Islamic Republic has been trumpeting its possession of the drones in an attempt to embarrass Washington over its alleged surveillance of Iran's disputed nuclear program.
Hajizadeh said Iran had previously acquired a ScanEagle drone and produced a copy of that, but did not provide evidence to back up the claim.
Last month, Tehran claimed that a U.S. drone violated its airspace. The Pentagon said an unmanned Predator aircraft came under fire at least twice while flying over international waters but was not hit.
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