A German military base near the western city of Cologne was temporarily closed on Wednesday as authorities investigated the possible sabotage of water supplies.

The Cologne-Wahn barracks employs around 5,500 people, including 4,300 soldiers and 1,200 civilians, according to German newspaper Der Spiegel.

A military spokesman, Ulrich Fonrobert, said that a hole was discovered in a fence leading to waterworks at the Cologne-Wahn barracks.

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

The barracks have now reopened although the drinking water system has been shut down as a precautionary measure, Fonrobert said.

“We are taking this incident very seriously; police, military police and military counterintelligence services (MAD) are investigating,” he added.

Also on Wednesday, the security level at a NATO air base in the western German town of Geilenkirchen was raised due to a suspected case of sabotage in a similar incident to that of the Cologne-Wahn base, Reuters reported. However, after checking the condition of the water, the air base was not sealed off, according to a NATO spokesperson.

It is unclear who entered the Cologne-Wahn barracks. Wednesday’s incident comes amid heightened concerns that Russia could be carrying out sabotage attacks across Europe.

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

The statement pointed to potential “sabotage, acts of violence, cyber and electronic interference, disinformation campaigns, and other hybrid operations.”