An Egyptian lawyer who represents the family of murdered Italian student Giulio Regeni was prevented from flying to a UN conference and detained by a special prosecutor, his supporters and a security source said on Wednesday.

Ibrahim Metwaly, who founded the Association of the Families of the Disappeared to search for Egyptians who disappear in suspicious circumstances after his son disappeared four years ago, himself went missing while heading for his flight to Geneva on Sunday, his supporters say.

His Association said he was taken from Cairo airport by airport security and he was not heard from until Wednesday when a state security prosecutor ordered him detained for 15 days on charges of “joining a group founded illegally”, judicial sources said.

There was no comment from the Interior Ministry. A security source said Metwaly was legally arrested and not subjected to any violations.

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Rights activists say Egyptian security forces resort to kidnapping government opponents and keeping them in secret jails where they can spend weeks, months, or years without charge. The authorities deny the accusation.

Italian graduate student Giulio Regeni, who was conducting research on Egyptian trade unions, disappeared in Cairo on Jan. 25 last year. His body was discovered in a ditch on the outskirts of the Egyptian capital on Feb. 3, showing signs of extensive torture.

Metwaly has acted as a lawyer for Regeni’s family in Egypt as they sought to identify his killers.

“I am disturbed by alleged reports that lawyer Ibrahim Metwaly was arrested … while he was about to fly to Geneva to address the UN Working Group on forced disappearances,” the chairman of the European Parliament’s Subcommittee on Human Rights, Pier Antonio Panzeri, an Italian, said in a statement.

Regeni’s murder has strained ties between Italy and Egypt, traditional Mediterranean allies with strong economic ties.

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Security and intelligence sources said Regeni was arrested the day he vanished and taken into custody. Egyptian officials have denied any involvement in his death.

Italy recalled its ambassador in April 2016 as it sought to obtain evidence from Egypt to solve the murder. Magistrates in Rome and Cairo have met half a dozen times over the past year, but no one has been charged.

A new Italian ambassador arrived in Cairo on Wednesday.