(CNN) — A German military base near the city of Cologne was temporarily closed on Wednesday as authorities investigated possible sabotage of the water supply.

According to the German newspaper “Der Spiegel”, around 5,500 people are employed in the Cologne-Wahn barracks, including 4,300 soldiers and 1,200 civilians.

Military spokesman Ulrich Fonrobert said a hole had been discovered in the fence surrounding the water pipe at the Cologne-Wahn barracks.

And further: “In addition, the barracks were closed because it could not be ruled out at that time that the person was still on the premises.” However, the perpetrators were not found “despite an intensive search.”

The barracks have now reopened, but the drinking water supply has been switched off as a precaution, said Fonrobert.

“We take this incident very seriously; the police, military police and the Military Counterintelligence Service (MAD) are investigating,” he added.

Also on Wednesday, security was increased at a NATO air base in Geilenkirchen, western Germany, following a suspected case of sabotage in a similar incident to the one at the Cologne-Wahn base, Reuters reported. However, the base was not sealed off after a check of the water condition, a NATO spokesman said.

It is still unclear who entered the barracks in Cologne-Wahn. Wednesday’s incident occurred against the backdrop of growing fears that Russia could carry out sabotage attacks across Europe.

In May, NATO said it was “deeply concerned about recent malign activities” by Moscow. The statement pointed to possible “acts of sabotage, violence, cyber and electronic interference, disinformation campaigns and other hybrid operations.”

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