And even if they were brought to trial, it is unclear whether they would be held responsible for the torture and killing of Mr. Regeni, who was in Egypt to research labor unions for his thesis at Cambridge University in Britain.

The National Security Agency officers followed Mr. Regeni after his arrival in Cairo, but Italian and American officials believe that one of Egypt’s two other major security agencies, both of which have close ties to President Abdel Fattah el-Sisi, may also have played a role in his death.

One of those agencies, Military Intelligence, was once led by Mr. Sisi and has grown immensely powerful since he came to power in a 2013 military takeover. The other, the General Intelligence Directorate, is led by a longtime loyalist of the president and is dominated by Mr. Sisi’s son Mahmoud el-Sisi.

The five suspects identified by Italian prosecutors occupy much lower positions in Egypt’s security apparatus. Nonetheless, the Italian announcement is at least a notable shift in the tone of an investigation that has always been troubled by delays and dead ends.

In March 2016, Egyptian officials faced accusations of a botched cover-up after the Cairo police shot and killed five men they had accused of killing Mr. Regeni. Italy rejected that explanation, even after the Egyptian police produced Mr. Regeni’s passport, which they said had been found in an apartment of one of the dead men, — indirect proof, Italian officials said, that the true culprits were with Egyptian security.

Italian patience with the dragging investigation appeared to run short last week, after a meeting in Cairo between Egyptian and Italian prosecutors.

The two sides discussed surveillance footage from the station where Mr. Regeni was believed to have disappeared in January 2016. After Egypt handed over the footage in May, Italian officials were dismayed to discover that the portions where Mr. Regeni might have been present had been erased.