ROME — An Italian doctoral student brutally killed in Cairo had been in the cross hairs of Egyptian surveillance services until the day of his disappearance, and he was murdered over research he had been carrying out into Egypt’s independent labor unions, a senior Italian prosecutor has said.

The remarks on Thursday by Rome’s chief prosecutor, Giuseppe Pignatone, came as Italians marked the two-year anniversary of the disappearance of the student, Giulio Regeni. Mr. Regeni vanished on Jan. 25, 2016, and his body was found on the outskirts of the Egyptian capital nine days later bearing unmistakable signs of torture — welts, burns and fractures.

Egyptian officials have repeatedly denied having had any hand in the death of Mr. Regeni, who moved to Cairo in 2015 to look into Egypt’s labor unions for his thesis at Cambridge University in Britain.

In an open letter published in several Italian newspapers on Thursday, Mr. Pignatone wrote that it was precisely that research that had been the “motive for the murder.” The prosecutor said it was important to reconstruct the “reasons that led him to go to Cairo and to identify the people he met in the academic world and in the Egyptian trade unions.”