Regeni, a 28-year-old postgraduate student at Cambridge University, vanished in Cairo in January 2016. His body was found almost a week later and a post mortem examination showed he had been tortured before his death.
There was no immediate comment from authorities in Egypt. Egyptian officials have repeatedly denied any involvement in Regeni's killing.
The five suspects are all members of the National Security Agency and include a general, two colonels and a major, the source said. They have been placed under investigation for allegedly kidnapping Regeni. No one has yet been named in connection with the killing itself.
Under Italian law, being placed under official investigation does not imply guilt and does not automatically lead to a trial.
Regeni disappeared on Jan. 25, 2016 - the fifth anniversary of the start of the 2011 uprising that ended the 30-year rule of Hosni Mubarak.
Intelligence and security sources told Reuters https://www.reuters.com/article/us-egypt-regeni-exclusive-idUSKCN0XI1YU in 2016 that police had arrested Regeni outside a Cairo metro station and then transferred him to a compound run by Homeland Security.
However judicial sources in Rome told Reuters last week that Italy was frustrated by the slow pace of developments in Egypt and had decided to press ahead with its own line of enquiry in an effort to move things forward.
Egypt's state information service said on Monday that Italy had sought Egypt's approval for listing "a number of Egyptian policemen" as suspects during a meeting of the prosecutors from the two countries last week.
It said such a request had already been rejected in the past because Egyptian law did not recognise the procedure for placing suspects under investigation before possible charges are laid.
It also cited a lack of solid evidence for the request, which, it said, was "merely based upon initial police inquiries".
The judicial sources in Rome said that amongst those placed under investigation on Tuesday was a colonel who had met Italian prosecutors during their first visit to Cairo in February 2016. He had assured them that local security forces had had nothing to do with the disappearance of Regeni, the same judicial sources said.
Regeni had been researching https://www.reuters.com/investigates/special-report/egypt-regeni Egypt’s independent unions for his doctoral thesis. Associates say he was also interested in alternatives to the long-standing domination of Egypt’s economy by the state and the military.
Both subjects are sensitive in Egypt. The military’s grip on the economy is a subject rarely talked about in a country that has been ruled almost entirely by military men since the overthrow of King Farouk in 1952. (Additional reporting by Aidan Lewis in Cairo, Writing by Crispian Balmer, Editing by Jon Boyle)
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