ROME (AP) — Italian prosecutors on Thursday formally put three high-ranking members of Egypt’s national security force and one police officer under investigation in the 2016 kidnapping, torture and killing of an Italian youth doing doctoral research in Cairo.

Rome Prosecutor Michele Prestipino told a session of a special parliamentary commission on the slaying of Giulio Regeni that the criminal probe was formally closed earlier in the day.

Four suspects are being investigated for alleged kidnapping and one of them also is being investigated for aggravated injury and aggravated murder, Prestipino said. The investigation against another member of Egypt’s national security force was shelved for insufficient proof, he told lawmakers.

The case strained relations between Italy and Egypt, an ally it needs in the crackdown on migrant trafficking across the Mediterranean and efforts to combat terrorism.

As late as last month, Egyptian prosecutors were insisting that Regeni's killer remains unknown.

Regeni's body was found on a desert highway several days after he disappeared in January 2016. He was last seen near a subway stop in Cairo, where he had been researching union activities among street vendors for his doctoral thesis in his studies at Cambridge University.

His mother later said his body was so mutilated that she was only able to recognize the tip of his nose. The torture marks resembled those resulting from widespread torture practices in Egyptian facilities, activists and rights groups have said. In late November, Egyptian prosecutors slammed the Italian investigators in their push to bring five police and intelligence officers to trial.

An Italian judge will review the evidence and decide on whether to indict and order a trial for any or all of the suspects. Italy allows trials of defendants in absentia.