Thieves emptied 120 vaults

The four - three Italian men and a Dutch woman - are being held in solitary confinement.

The exact value of the gems stolen in the raid has not been revealed, but more 120 of the 160 vaults at the Antwerp Diamond Centre were emptied.

An earlier robbery involving only five vaults resulted in estimated losses of $4.5m.

They are being held on suspicion of being co-authors in the theft Leen Nuyts Antwerp Prosecutor's Office

Leen Nuyts, a magistrate at the Antwerp prosecutor's office, said a court would decide on Thursday what would happen to the detainees.

"They are being held on suspicion of being co-authors in the theft," she said.

No further information about the four was released, but Ms Nuyts said their arrest was based on "developments in the ongoing investigation".

Many of the traders whose diamonds were taken are abroad, and have still not provided police with details on the contents and their value, Ms Nuyts added.

'Inside help'

The Diamond Centre building, located in the heart of Antwerp's historic diamond district, is closely guarded.

There are surveillance cameras, entry codes, 24-hour security guards, and even cameras in the vaults.

With no signs of a break-in, police suspect the thieves could have had inside help.

They have been questioning staff and owners of the safes at the centre.