CAIRO — An Italian prosecutor flew to Rome from Cairo this week bearing precious cargo: a thumb drive containing surveillance video from the Cairo metro system that could hold the key to a politically charged murder investigation.

The Italians hope the images will lead to the killers of Giulio Regeni, 28, an Italian graduate student whose battered body was found on a Cairo roadside in February 2016. Mr. Regeni’s death traumatized Italians, led to a 16-month diplomatic freeze between Egypt and Italy, and produced heated accusations that Egypt was withholding evidence because members of its security services were among the chief suspects in the case.

On Wednesday, over two years after the killing, the Egyptians handed over surveillance footage from the subway system where Mr. Regeni is thought to have vanished. Italian specialists will use facial recognition software to scan video fragments from hundreds of cameras for any glimpse of Mr. Regeni, who had come here to study the trade union movement.

But their chances of success are slim. By the time Egyptian authorities recovered the video, more than a week after it had been requested by the Italians, most of it had been erased, overwritten by later surveillance recordings. Just 5 percent of the images from the night Mr. Regeni disappeared remain, an Italian judicial official said, speaking on the condition of anonymity.