Ivo Sanader, Croatia's prime minister, sacked the country's interior and justice minister earlier this month amid concerns that organised crime is spiralling out of control in the capital.
"This is a shock, a disaster... The police and all of us have to do everything to stop this violence. This has to stop," Zlatko Mehun, a government spokesman, said.
EU accession threatened
Croatia is bidding to join the European Union but organised crime and corruption is threatening to derail accession talks with the 27-member bloc.
Davor Butkovic, an editor of Jutarnji List, a daily Croatian publication, said the latest attack in Zagreb would damage the government’s attempt to join the EU.
"Unfortunately, this means that the state has lost this round of crackdown on crime. This is a big blow to Croatia's political system; it shows the system's inefficiency in fighting crime."
Pukanic, who become known for uncovering a series of corruption and human rights abuses, earlier this year reported an assassination attempt against him.
Earlier this month, the daughter of a lawyer was shot dead in the building where she lived, close to Zagreb’s police headquarters.
Ivana Hodak's father Zvonimir is defending an ex-general who is suspected of stealing $5m in diamonds during Croatia's 1991-1995 war of independence
Ivana Hodak had once dated a lawyer whose clients had been tried for organised crime.