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

"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 official Italian response, despite the clamour 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.

Italy's state-run RAI and Italian politicians questioned the latest Egyptian scenario and demanded the truth.

"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, miraculously found, rather than immediately getting rid of such a shocking proof of their crime."

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.

"The Italian government and prosecutors shouldn't give any credibility to what seems like a false reconstruction," he was quoted as saying by the ANSA news agency.

The doubts weren't confined to Italy.

In Egypt, prominent activist Wael Ghoneim, one of the top activists of the uprising and an anti-police torture advocate, said the Egyptian theory was "very cute."

"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 of victims from other 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 contradiction 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 and foreign media about Regeni's death ever since he disappeared. 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 police and 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, especially a foreign student researcher, and noted that no ransom demand was made.

"I think it (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.

AP writers Maggie Michael and Sam Magdy in Cairo contributed