CNN  — 

Four security guards are under investigation over their suspected involvement in an audacious heist from a vault in the German city of Dresden, prosecutors have confirmed.

On November 25 several thieves gained access to the Green Vault at Dresden Castle, containing one of the largest collections of art treasures in Europe, and lifting around 100 priceless pieces.

Now prosecutors have revealed that a private individual brought criminal charges against two security guards who were on duty at the time, and who have been accused of not responding adequately to the theft, nor doing enough to prevent it.

The prosecutor’s office launched investigations into two other guards, who were suspected of assisting the robbers.

The first was arrested on suspicion of providing information on the Green Vault and its security systems to the thieves, and police searched their apartment on November 29.

© Staatliche Kunstsammlungen Dresden
The Dresden Castle is a residential palace incorporating baroque, Renaissance and classical styles. Today it houses a complex of great museums, including the Green Vault.
© Staatliche Kunstsammlungen Dresden
The inner courtyard of Dresden Castle is sheltered by a curved lattice glass roof designed by local architect Peter Kulka.
© Grünes Gewölbe, Staatliche Kunstsammlungen Dresden, Photo: David Brandt
For Augustus the Strong, the Mughal emperor Aurangzeb was an idealized figure of absolute power and limitless wealth. This jewel-encrusted diorama of the Mughal court cost Augustus more than the construction of the Moritzburg Castle itself.
Courtesy Dresden State Art Collections
The 41-carat Dresden Green is the largest and finest natural green diamond in the world. In 2000 it was displayed at the Smithsonian next to its blue counterpart, the Hope Diamond.
Courtesy Dresden State Art Collections
This gold and tortoiseshell notebook was created by goldsmith Pierre Triquet and jeweler Johann Heinrich Köhler. The central focus is a heart-shaped opal topped by a ruby, while the large diamond above serves as a clasp.
Courtesy Dresden State Art Collections
The Order of the Golden Fleece was the most prestigious and exclusive award in the House of Habsburg. This medal crafted by Johann Melchior Dinglinger features three flaming rubies surrounded by 70 diamonds.
Courtesy Dresden State Art Collections
An epaulette of the Dresden Court showcases over 200 rose-cut diamonds in a double loop motif.
© Staatliche Kunstsammlungen Dresden
This exquisite frigate of ivory and gold, supported by a figure of Neptune, was the final work of the noted Dutch carver Jacob Zeller. The ship's billowing sails are wafer-thin slivers of ivory.

No relevant items were found and the individual was released without charge on November 29, prosecutors said.

Investigators suspected the second guard may have interfered with the alarm system, thereby assisting the theft. Their apartment was also searched.

Again nothing was found, but the investigation is ongoing, prosecutors said.

“The two parties concerned cooperated and initially announced that they wanted to provide information on the matter, but then exercised their right to remain silent,” prosecutors said in a press statement sent to CNN Wednesday.

“No formal questioning has taken place so far.”

Items of “immeasurable value” were stolen during the heist, said local politician Roland Woeller at the time. “This is an attack on the cultural identity of all Saxons and the state of Saxony.”

The vault features an astounding collection of historical jewelry and precious ornaments, including bowls carved out of crystal and agate, diamond-encrusted brooches, jeweled figurines and goblets fashioned from gilded ostrich eggs.

One of the most famous pieces of the collection, a 41-carat green diamond known as the Dresden Green, was not in the museum at the time. It is currently on loan to the Metropolitan Museum of Art in New York City.

CNN’s Bianca Britton and Ivana Kottasová contributed to this report.