Croatia on Wednesday handed over Stretko Kalinic, convicted in connection with the assassination of prime minister Zoran Djindjic, to Belgrade, Serbian television RTS reported.
He was flown from Zagreb on a Serbian government plane and sent to an unnamed "holding point" under high security, the report said, DPA reported.
A Belgrade court sentenced Kalinic in 2007 in absentia to 30 years in prison for his role in the 2003 murder of then-premier Djindjic and, in a separate trial, to 40 years as a lead hitman for a powerful crime group.
He had been on the run since 2003 until another Serbian criminal shot and wounded him in the Croatian capital in June. Belgrade authorities hope that Kalinic may shed additional light on the background of the Djindjic assassination.
Criminals and rogue police commandos were sentenced for the assassination, but, despite allegations at the time, no politician, judge or high-ranking police official.
The extradition was the first-ever of a Croatian national - Kalinic holds both Croatian and Serbian passports - to Serbia under an agreement Zagreb and Belgrade signed in June to allow the handover of nationals involved in organized crime.
Other former Yugoslav republics are expected to sign similar treaties in order to remove the shelter criminals enjoy from arrest and trial by laws that ban the extradition of nationals.