Although in the movie it is hard to tell, the book places him in this spot during his period as a Death eater. Popular consensus is that it is Him. The reason you never find out, or see the mask is to hide his identity as a double spy for Albus.
Yeah, that's where I got the info in the first place.
This death eater is seen during the attack in the Department of Mysteries in Order of the Phoenix.
I'm having a little trouble believing it's Snape, although I must admit it is definitely a possibility
It's not snape (as far as the film verion is concerned at least) as to the best of my knowledge Alan Rickman never had a Death Eater costume - if he did, it would almost certainly have leaked or been intentionally publicised via the PR pics.
Looking at the robes, my current feeling is that it was either MacNair, of, if you look at this pic, the DE on the left:
Id guess that its the second from the left, or the guy on the very right - The mask has blue eyes in it. Blonde guy should have blue eyes, then sthe second from the left may have blue eyes. Maybe rewatch the scene and try and match the eyes hehe.
The thing is, there's lots of speculation here.
We definitely have some clues to go by, but at present there's nothing solid to 'match the mask to the man'
Watching the movie, whenever the mask is seen the rest of the costume is not illuminated well enough for us to see.........................................