I think that is a lie to cover up what they had done.
All the things Iran has been doing since like bulldozing the site, saying the black boxes have been damaged, might not be any good when there were pictures of the boxes and were only a little scorched, but by now they have probably taken sledge hammers to them.
They are GUILTY of bring down a passenger plane.
The question is WHY?
Check the names on the planes manifest against the 30 members of RG that were arrested.