Italian prosecutors are investigating whether the wrong man was extradited to Italy in a major operation against people-smuggling.
Mered Medhanie is thought to be at the heart of an empire trafficking migrants from Africa to Europe.
A 35-year-old Eritrean man said to be Medhanie was held in Sudan in May and flown to Rome on Tuesday.
But since then, friends and family of the man have told the BBC there has been a case of mistaken identity.
Investigators say Medhanie – dubbed The General – was responsible for the death of 359 migrants who drowned when their boat sank off the Italian coast in October 2013.
The man who was extradited was arrested by Sudanese police with the help of British and Italian authorities, Britain’s National Crime Agency (NCA) said.
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But Francesco Lo Voi, the chief prosecutor in the Italian city of Palermo, now says “we are carrying out the necessary checks”.
“The identification of the suspect, his arrest, his handing over and his extradition to Italy were communicated to us in an official manner by the NCA and the Sudanese authorities through Interpol,” he told the Ansa news agency.
The NCA said it was “too soon to speculate” about the claims, but added it was “confident in its intelligence-gathering process”.
Friends of the detainee identified him as Mered Tesfamariam, a 27-year-old migrant.
A woman living in Norway who identified herself as his sister told the BBC’s Newsnight programme her brother was “completely innocent”.
Hiwet Tesfamariam Berhe Kidane said she last spoke to him about three weeks ago and first learned of his arrest when she saw pictures of him on television and social media.
Hermon Berhe, who lives in Ethiopia, said he grew up in Eritrea with the man shown in pictures handed out by Italian police.
“I don’t think he has any bone in his body which can involve such kind of things,” he said. “He is a loving, friendly and kind person.”
And Meron Estefanos, a Swedish-Eritrean journalist who interviewed the real Mered Medhanie in a phone call last year, said: “I called refugees who know the real smuggler and I showed them the picture of what the Italians published and then everybody said, ‘No, that’s not the smuggler that smuggled us into Europe’.
“I believe they have the wrong person. This is a refugee who happened to be in [Sudanese capital] Khartoum at the wrong place at the wrong time.”
She added that rumours were going around Khartoum that the real Medhanie had been arrested in Sudan but was able to bribe himself out of jail.