Citing sources close to the Serbian Interior Ministry, who spoke on condition of anonymity, the daily reported that Simovic, kept in detention after he was arrested by the Serbian police while trying to illegally cross from Croatia to Serbia, said that he knew where the head of Cvetko Simic was buried.
Simic's dismembered body was found in Jarun Lake in Zagreb this March.
Simovic is believed to have seriously wounded another mobster Sretko Kalinic in a shootout near Rakitje Lake outside Zagreb in early June. Kalinic was admitted to hospital in the Croatian capital and Simovic was nabbed by the Serbian police two days after the incident while trying to illegally enter Serbia.
According to the daily, Simovic and Kalinic are suspected of Simic's murder.
Simovic reportedly offered information on that murder in exchange for the status of collaborator of justice during talks with Serbia's deputy special prosecutor Jovica Jovanovic and two interior ministry officials who visited him in custody.
Both Kalinic and Simovic were tried in absentia for their involvement in the 2003 assassination of PM Djindjic and were given each 30 years in prison. They had been on the run since then.
Commenting on the extradition of Kalinic, who also holds Croatian citizenship as he was born in southern Croatia, Croatian Justice Minister Ivan Simonovic has recently said that Serbia's comprehensive request for his handover has been forwarded to the the Zagreb County Court which will decide on the matter.
Last Tuesday Simonovic and his Serbian counterpart, Snezana Malovic, signed a bilateral extradition agreement which will facilitate extradition of organised crime and corruption convicts.
On Friday, Croatian Interior Minister Tomislav Karamarko dismissed as "ridiculous prattle" media speculations that Serbian mobster Cvetko Simic was killed for allegedly working for Croatian secret services.
Minister Karamarko said, while responding to reporters' questions, that such speculations appeared because some newspapers needed headline material.
Simic had criminal records in Serbia for drug smuggling and trafficking.