The thieves who broke into the Dresden Green Vault in Germany four months ago, making off with $1.1 billion in precious gems, may have had inside help.

Four Dresden Green Vault security guards are under investigation in connection with the Nov. 25 overnight break-in, The Art Newspaper reported online Tuesday.

Jürgen Schmidt, a spokesman for the Dresden prosecutor’s office, said that two of the four guards were under investigation because of “an action concerning the alarm system that may have benefited the thieves,” according to German media outlets.

DRESDEN MUSEUM HEIST THIEVES STEAL $1.1 BILLION IN ‘PRICELESS’ JEWELS

German press agency DPA reported that one of them was arrested in November on suspicion of passing documents about the Green Vault’s layout and security system to the thieves, only to be released after a house search turned up no relevant evidence, according to the newspaper.

DPA reported that the other two guards were on duty when the break-in happened. They were accused in a complaint lodged by an individual of not taking quick action to prevent the theft, the Art Newspaper reported.

“The suspects have behaved cooperatively and initially said they wanted to speak to investigators, but subsequently they reserved their right to silence,” Schmidt told the Bild newspaper, according to the Guardian.

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Last week German police reported that at least seven people took part in the heist.

Prosecutors and police said they have determined that an Audi S6 used in the theft and later set alight in a Dresden garage was sold to an unidentified buyer in August. They said they believe a young man who picked up the car from the seller in Magdeburg, another eastern German city, was connected to the break-in and released a sketch of a slim dark-haired man believed to be about 25.

A large diamond brooch, a diamond epaulet and other treasures were taken by the robbers.

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The Green Vault is one of the world’s oldest museums. It was established in 1723 and contains the treasury of Augustus the Strong of Saxony, comprising around 4,000 objects of gold, precious stones and other materials.

The Associated Press contributed to this report.