A letter bomb has been delivered to the headquarters of Eurojust, the EU agency which works to boost judicial co-operation.
All the devices have been defused with no injuries
The device, which did not explode, was the fourth letter bomb delivered to a European figure in the past three days.
European Commission President Romano Prodi escaped unhurt when the first such device exploded at his home in the Italian city of Bologna on Sunday.
A previously unknown Italian anarchist group is suspected of being behind the wave of attacks.
On Monday, similar devices were delivered from Bologna to European police body Europol and the European Central Bank chief, Jean-Claude Trichet.
The Hague prosecutor's office said the most recent suspicious package was found at the offices of Eurojust on Tuesday morning.
Eurojust and nearby offices at the International Criminal Court were evacuated while a team of explosives experts investigated the package.
Eurojust is the EU body which helps co-ordinate investigations among European police forces.
The ECB's Jean-Claude Trichet received a bomb a day after Prodi
Dutch prosecutors said they were investigating whether the same person was behind all the devices.
German police said they were focusing, with their Italian counterparts, on a group called the Informal Anarchist Federation.
It has admitted carrying out two other explosions in rubbish bins near Mr Prodi's home in the past week.
Targeting 'new order'
In a letter to an Italian newspaper the group said it wanted to target "the apparatus of control that is repressive and leading the democratic show that is the new European order".
Mr Prodi said the package he received at home contained a book wrapped in yellow paper and addressed to his wife, which been cut to insert explosive powder.
The book used was The Pleasure by Gabriele D'Annunzio, a famous early supporter of Fascism before his death in 1938.
Mr Prodi joked that the choice of book might have been ironic.