"People didn't know what do to do with him so they let him go. But they took his identity card. There are people there who know who he is. Some people were also taking photos of him."
So they have his ID card and some photos of him after he was apprehended by the crowd - plenty of evidence to refute the Iranian government's official claim that the shooter was a member of 'an opposition group'.
The best refutation of that is the fact that government banned a proper funeral and memorial service for Neda. That amounts to a murder confession by the mullahs.