(JTA) — The oldest surviving Jewish cemetery in Europe was vandalized.

At least 50 gravestones in the medieval Jewish cemetery in the German city of Worms were smeared with a greenish paint, the city said in a statement.

The incident took place on Thursday. The cemetery was ordered closed for a week.

There are about 2,500 gravesites in the cemetery, which is called Heiliger Sand, or Holy Sands, some dating back to the mid-11th century. Among the vandalized gravesites was the tomb of Rabbi Meir of Rothenburg, known as the Maharam, a prominent rabbi who died in 1293.

The city website said that it did not believe the vandalism was motivated by anti-Semitism or politics.

Meanwhile, experts reportedly are working to figure out how to clean and restore the gravestones.

“We do not yet know what material the paint is made of and how we can remove the smears without damaging the valuable tombstones,” Mayor Hans-Joachim Kosubek said in the statement “According to initial findings, the main gravestones in the medieval part of the cemetery are affected, the rocks of which are particularly sensitive due to their age.”

The statement also called the incident a “slap in the face” to the city.

The cemetery played a central role in a bid for the cities of Worms, Speyer and Mainz to be placed on the list of UNESCO World Heritage sites. The city noted that the cemetery attracts thousands of Jewish visitors each year.