Four guards are under investigation in relation to the audacious Green Vault heist at Dresden Castle in the Germany city last November, according to prosecutors.

Historical treasures were briskly snatched in the predawn heist, and since then two security guards who were on duty have come under scrutiny for not doing enough to prevent the theft. A private individual has brought criminal charges against these two guards.

Another two guards are under suspicion of assisting the robbers. Their apartments were searched in November, but no evidence was found.

Several of the stolen pieces had been collected by Saxony’s 18th-century ruler, Augustus the Strong.

Germany’s culture minister, Monika Grütters, said the theft was “appalling and shocking.” She added, “This robbery of pieces which define our identity as a nation of culture breaks our hearts.”

The Green Vault's lauded Dresden Green Diamond and other signature treasures were not on display at the time of the theft since it was on loan to the Metropolitan Museum of Art in New York for the exhibition Making Marvels: Science and Splendor at the Courts of Europe.