ROME (AP) — Italian prosecutors on Thursday formally put four high-ranking members of Egypt’s security forces under investigation over the 2016 kidnapping, torture and killing of an Italian doctoral research student in Cairo.

Rome prosecutor Michele Prestipino told a special parliamentary commission on the slaying of Giulio Regeni, 28, that his office formally closed the criminal probe earlier in the day, paving the way for possible trial indictment of the suspects, including in absentia.

The 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 due to insufficient proof, he told lawmakers.

The case strained relations between Italy and Egypt, an ally in the crackdown on migrant trafficking across the Mediterranean and in efforts to combat terrorism. At one point, Italy withdrew its ambassador to protest perceived reluctance by Egypt to reach the truth.

Prosecutors have indicated that Egyptian security officials suspected Regeni was aiming to foment a revolution, and might have been working for U.S. or Israeli intelligence.

As late as last month, Egyptian prosecutors were insisting that Regeni's killer remains unknown. Egyptian authorities have alleged that the Cambridge University doctoral student fell victim to ordinary robbers.

“The Rome prosecutor's office has stuck firm to the commitment to do everything to ascertain every responsibility,” Prestipino told the commission. “We owed it to the memory of Giulio Regeni and to being magistrates of this Republic.”

He told the lawmakers, many of whom have insisted Rome should keep up diplomatic pressure on Egypt for judicial cooperation: "We contend that we have obtained unmistakable and significant elements of proof of the responsibilities of those under investigation.”

Regeni's body was found on a desert highway days after he disappeared on Jan. 25, 2016. He was last seen near a subway stop in Cairo, where he had been researching union activities among street vendors for his studies.

His mother has said her son's body was so badly mutilated that she was only able to recognize the tip of his nose when she viewed it. The torture marks resembled those resulting from widespread torture in Egyptian facilities, activists and rights groups have said.

On Thursday, prosecutors told the lawmakers that Regeni was tortured with cutting instruments, burned and clubbed during the nine days from when he was kidnapped to when he died.

Claudio Regeni, the young man's father, renewed the family's appeal to the Italian government to once again withdraw its ambassador in a sign of protest.

Paola Deffendi likened the details that emerged from the investigation of her son's murder to a “mirror of how human rights are violated in Egypt every day.”

In late November, Egyptian prosecutors slammed the Italian investigators in their push to bring five police and intelligence officers to trial.

The Italian prosecutors said several witnesses, whom they identified before the parliamentary panel only by code names to protect their security, were invaluable.

According to one witness, two or three days after Regeni's disappearance, the young man was in “Room No. 13” of a villa that is part of the interior ministry. Regeni was chained in irons, his upper body nude and showing signs of torture, according to the witnesses account, described by the prosecutors. Regeni was speaking deliriously in Italian, the witness said.

Another described being in the police station the evening that Regeni disappeared and seeing a young man speaking Italian and asking for a lawyer or to get in touch with the Italian consulate. The youth had arrived accompanied by four people in civilian clothes.

The same witness recounted that someone else at the police station, also under arrest, tried to help Regeni, but was elbowed in the face by a police officer.

The Egyptian suspects can supply their own statements or other information in the next few weeks. Then 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.

Italian prosecutors named as the suspects in the kidnapping Police Maj. Sherif Magdy; Police Maj. Gen. Tareq Saber, who was a top official at the domestic security agency at the time of Regeni's abduction; Col. Hesham Helmy, who was serving at a security center in charge of policing the Cairo district where the Italian was living, and Col. Acer Kamal, who headed a police department in charge of street operation and discipline.

Magdy served at the same agency as Saber and headed the team which allegedly put Regeni under surveillance. Magdy is under investigation for the injuries inflicted on Regeni and his murder as well as the abduction, prosecutors said.

Saber retired in 2017. The investigation was shelved against junior police officer Mahmoud Nejm.

“I don't know the Egyptian legal code, but I know the Italian legal code, and I know we have sufficient elements” to prosecute the suspects, Prestipino said.

Fellow Rome prosecutor Sergio Colaiocco said that the witnesses included one who told Italian investigators that, two days before Regeni disappeared, police came to where the young man was living and searched his room, intimidating the witness to keep quiet about the search.

According to another witness account, a national security officer came more than a month before Regeni was abducted to the place where the Italian was living and also told the building doorman not to mention the visit, prosecutors said.