They made a macabre face mask from his skin, cooked his flesh and ate him for lunch. Later they threw the bones into the Manzanares river in the Spanish capital, Daily Mail reported Friday.
Fellow gang member Sretko Kalinic, nicknamed The Butcher, confessed to the crime after being arrested in the Croatian capital Zagreb last year.
Spanish officers this week found 50 bones in the river and located the house of horrors where Milan Jurisic, 37, was killed.
Jurisic is thought to have betrayed his fellow gang members by stealing money from them.
He was on the run after being convicted in his absence of assassinating then Serbian prime minister Zoran Djindjic in 2003.
Spanish police believe the cannibal gang who killed him were led by Luka Bojovic, arrested in Valencia last month and also accused of assassinating Djindjic.
All of those involved in the gruesome Madrid murder are said to be members of the Zemun Clan, a mafia group from Belgrade.
The crime, thought to have been committed in 2009, is being investigated by magistrate Fernando Andreu at the National Court in Madrid, according to the newspaper.