The Rome trial in absentia of four Egyptian security officials accused of torturing and killing Italian student Giulio Regeni has been suspended.

Judges referred a constitutional question raised by the defence to Italy’s Constitutional Court on Thursday. The issue concerns the defendants’ right to defence and the appointment of expert witnesses, judicial sources said.

The court ruled the matter was “not manifestly unfounded” and “relevant” to the case, prompting the suspension.

Regeni, a 28-year-old Cambridge University doctoral researcher from Friuli, disappeared in Cairo on January 25, 2016. His half-naked body was found on February 3 beside the Cairo-Alexandria highway, showing clear signs of torture.

Prosecutors allege he was abducted and murdered by National Security General Tariq Sabir and his subordinates, Colonels Athar Kamel Mohamed Ibrahim and Helmi, and Major Magdi Ibrahim Abdelal Sharif.

Egypt has refused to notify the four officials of the Italian proceedings, so they remain absent from the trial.

Regeni was reportedly targeted because of his research on Cairo street unions, considered politically sensitive by Egyptian authorities. Investigators believe he was accused of being a spy after a street vendor union leader falsely identified him.