iPhoto thumbnails disappear - missing photos

iPhoto crashed a few days ago, and I had to rebuild the library. After this was done, my events were all out of order but the photos were still in tact. However, there were about 100 photos in one event that were missing. They were represented by black squares. The thumbnails would show for an instant while I was scrolling through the event, but then the thumbnails would disappear and turn black. When I selected a photo, it showed a black exclamation warning sign.

I back up to Time Machine, so I tried to restore my iPhoto library from an older backup. Those same 100 photos seem to be corrupt in every single backup since the photos were taken. To make it even harder for me to understand, I actually viewed these 100 photos in iPhoto last week and they were fine, so I know they haven't been corrupt since they were imported to iPhoto.

Are these photos recoverable?