ROME (AP) — Doubts mounted Friday over Egypt's claims that members of a criminal gang were linked to the torture and death of an Italian doctoral student, amid continued speculation that Egyptian police themselves were involved.

"I'm sorry, I don't buy it," tweeted former Italian Premier Enrico Letta.

Egypt's Interior Ministry said Thursday that police found ID cards and other personal belongings of Giulio Regeni during a search of a house connected to a gang that specializes in abducting foreigners while posing as policemen. Four gang members were killed in a gunfight, the statement said.

There was no immediate response from the Italian government, despite the clamor that the Regeni case has sparked since the 28-year-old researcher disappeared Jan. 25, the fifth anniversary of the 2011 uprising when police were heavily deployed across Cairo. His body was found nine days later bearing signs of torture.

The Egyptian ministry issued a subsequent statement Friday saying the probe into the "criminal ring" was continuing in coordination with Italian investigators.

Rome's chief prosecutor, Giuseppe Pignatone, said the information released so far was "inadequate" to determine the circumstances of Regeni's death or identify his killers, the ANSA news agency reported. He said he was waiting for more evidence from Cairo.

Italian officials have repeatedly complained about a lack of transparency from Cairo in the investigation amid media speculation that Regeni might have been victim of the widespread torture and secret detentions by police that have been denounced by rights activists.

Egyptian officials deny that police were behind his death.

"We are hurt and embittered by the latest attempt of diversion by Egyptian authorities," Regeni's parents said in a statement to ANSA, demanding the Italian government respond.

Lawmaker Francesco Ferrara, a member of parliament's Copasir security committee, said the Egyptian theory left too many questions unanswered, including why Regeni had been detained for days before being killed.

"There's no explanation for why ordinary criminals, whose alleged objective was a robbery or ransom, would have inflicted such cruelty that is used only by torture professionals," said Pia Locatelli, head of the lower chamber's human rights committee. "And there's no explanation why they would have kept his documents."

The doubts weren't confined to Italy. In Egypt, prominent activist Wael Ghoneim criticized the Egyptian theory.

"So after they kidnapped Regeni and tortured him to death, they kept his passport, university ID in their house as a souvenir," he wrote on Facebook.

Rabab el-Mahdi, one of Regeni's friends and a university professor, said she was "sad, angry, and speechless regarding the recent killing of four people on the pretext that they killed Giulio."

"This sad excuse for a government just decides to murder every time they are cornered," she wrote on her Facebook page.

Maj. Gen. Said Shalaby, police chief in Qalubiya, where Regeni's belongings were found, said the documents were discovered "by accident" during a search after the gang shootout that also turned up bank statements and cash from robberies.

"We didn't know that these are the people behind Regeni's killing," he told The Associated Press.

Shalaby said police found the documents inside a car; the official Interior Ministry statement said they were found in the house. The discrepancies couldn't immediately be explained.

Ahmed Nagy, the chief prosecutor in Regeni's case, told AP that prosecutors were investigating the police claims and would question two women who testified that Regeni's personal possessions belonged to one of the slain gang members.

The Interior Ministry's statement was the latest in a string of theories that have been floated in the pro-government Egyptian media about Regeni's death. Investigators initially said he had died in a car accident. Then Egyptian media suggested he was gay or that he had a fight with another Italian a day before he died.

"The allegation that a criminal gang abducted, tortured and killed Giulio is inherently implausible and unlikely," said John Chalcraft, a professor of government at the London School of Economics, who has given expert witness testimony to the British courts about Egypt's police and security forces.

He said there was no previous known case of an Egyptian criminal gang targeting a foreigner for ransom and noted that no ransom demand was made.

"(The theory) lacks credibility, it doesn't stand up to scrutiny and wouldn't pass any test that would be applied by any serious international investigation," he said.

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AP writers Maggie Michael and Sam Magdy in Cairo contributed.