As Iran mulls retaliation for nuclear scientist's death, a riddle remains. What exactly was he working on?

Crushed flowers in the fresh snow. Pockmarked asphalt, fences flattened apparently by a blast wave, and bullet holes in the curbside. Bar the huge poster of assassinated Iranian nuclear scientist Mohsen Fakhrizadeh that sits above the spot where he was gunned down, few traces remain a week on of the killing that has rocked Iran and the region.