Croatia's government launched a crackdown after Mr Pukanic's murder
Four men have denied murdering the influential Croatian journalist, Ivo Pukanic, in 2008, at the start of their trial in the capital, Zagreb.
The four pleaded not guilty. Two others are being tried in their absence.
The editor of the Nacional magazine was killed alongside his marketing manager, Niko Franjic, by a bomb in a suspected organised crime attack.
Mr Pukanic was known for investigations into corruption and crime at the highest levels of Croatian society.
The trial is being held under maximum police security, with bodyguards assigned to the prosecutor and judge.
Streets around the Zagreb county court were sealed off and local media said snipers were posted on nearby buildings.
Details of the injuries sustained by Mr Pukanic and Mr Franjic were revealed for the first time in the courtroom. They included broken skulls and shattered kidneys.
The four who appeared in court on Wednesday were Robert Matanic, Luka Matanic, Amir Mafalani and Slobodan Djurovic.
Mr Pukanic had run a series of articles about organised crime in the region
Not guilty pleas were entered for two others.
One of them, Serbian citizen Bojan Guduric, surrendered to the authorities in Bosnia on Tuesday, the day before the trial began.
He has reportedly asked to be extradited to Croatia.
The other, Zeljko Milovanovic, is in custody in Serbia - which does not extradite its citizens.
Mr Milovanovic is one of three suspects arrested in Serbia over the case.
Another of those in custody in Serbia is Sreten Jocic, an alleged high-ranking organised crime member alleged to be behind the killing.
Ivo Pukanic swam in dangerous waters, says the BBC's Mark Lowen in Zagreb.
He published frequent investigations into the criminal underworld of the Western Balkans, exposing smuggling and corruption at the highest levels.
He was thought to have close links with some criminals himself, but it is widely believed he fell victim to an organised crime attack and was targeted for what he wrote.
The murder stunned Croatia, revealing the grip of organised crime and corruption.
The authorities are under serious pressure to tackle these issues if Croatia is to fulfil its aim of joining the European Union by 2012.