The former commander of a Serbian special police unit led the group which organized the assassination of Serbian Prime Minister Zoran Djindjic, the government said in a statement this evening.
Mr Milorad Lukovic, better known as Legija, former commander of a special police unit, was among twenty people suspected by the government, the statement said.
"The assassination on Serbian Prime Minister Zoran Djindjic was an attempt by this group to halt the fight against organized crime," the statement said.
It said the alleged gang had "tried to spark chaos and fear in Serbia" by killing the prime minister.
Mr Djindjic (50) died in hospital this afternoon after he was shot in the chest.
Mr Djindjic a reformer who played a large part in the downfall of former Yugoslav President Mr Slobodan Milosevic, was shot in the chest by two large calibre sniper bullets fired from a distance.
The shooting took place outside the main government building in Belgrade.
He narrowly escaped injury in another incident last month when a truck suddenly swerved out of its lane towards a convoy of cars - one of which was carrying the prime minister.
Police cordoned off the building after the shooting took place around 11.45 (Irish Time). Government ministers immediately gathered in emergency session, a government source said.
Jailed as a dissident student in the 1970s, frustrated as a popular protest leader in the 1990s, Mr Djindjic rebounded in a lightning street uprising in 2000 to become leader-in-waiting of a new democratic Serbia.
Djindjic, who spoke German and English, was married with two children.
The Taoiseach, Mr Ahern, today paid tribute to the assassinated leader.
"Zoran Djindjic was a courageous political leader, who made an immense contribution to reform and democracy in Serbia in recent years," Mr Ahern said.
Former Yugoslav president and longtime rival Mr Vojislav Kostunica, said he was shocked by the attack.
"The fact that political violence is happening... is a terrible warning about how little headway we have made on the path of real democratisation of our society," he said.
A state of emergency has been called and the Serbian cabinet announced three days of mourning.
Additional reporting