Police in Serbia say they have arrested the deputy public prosecutor, Milan Saraljic, during their continuing search for the killers of Prime Minister Zoran Djindjic.
The Serbian authorities are in the midst of a massive crackdown
Reports say the police allege Mr Saraljic has links with a criminal gang they suspect of being involved in the killing.
He is the first senior official to be arrested over the case.
Earlier, the Serbian Government ordered 35 judges to retire, including seven from the Supreme Court.
No specific allegations have been made against them.
Restrictions on local media are also being enforced, with two dailies - Nacional and Dan - and the weekly Identitet banned on Tuesday allegedly for hindering the police investigation.
In a separate development, Serbian Interior Minister Dusan Mihajlovic said police now knew the identities of two of the three people who carried out the assassination.
He added that they were members of the Zemun clan, an organised crime network with alleged links to supporters of former President Slobodan Milosevic which is said to have organised the killing.
The photo of the third man was published in several newspapers on Wednesday.
Zivkovic: More than 700 have been arrested
It has been reported that he may be a citizen of a neighbouring country.
Mr Djindjic was shot by sniper fire outside the government headquarters in central Belgrade.
The three are said to have carried out the attack from the second floor of a nearby building.
The BBC's Nick Hawton in Belgrade says the authorities are in the midst of a massive operation against those they believe have links to organised crime in Serbia.
The new Prime Minister, Zoran Zivkovic, has said more than 750 people have been arrested in the police inquiry since Mr Djindjic's assassination last week.
But he said in parliament on Tuesday that the assassination of Mr Djindjic had a clear political background and was not linked solely to the mafia.
Under the emergency powers, people can be held for 30 days without charge, and without access to lawyers.
Several members of the Zemun clan are still at large, including leader Milorad Lukovic, nicknamed Legija, a former paramilitary commander in the wars in Croatia and Bosnia.