European School I in Kirchberg was closed to pupils and staff on Monday, following the discovery of a specific written threat on a toilet door.
The message threatened that a shooting would take place at the school that day, reportedly stating: "Be ready."
In response, heavily armed police officers were stationed outside the school, while other officers and a sniffer dog were deployed inside the building from as early as 6am, according to RTL sources.
School security staff turned away everyone who arrived at the premises on Monday morning, ensuring that no one entered as a precautionary measure.
Parents told RTL that they received an email on Monday informing them that the school would remain closed for the day due to the incident.
A spokesperson for the police confirmed that an operation was underway at European School I, while school officials were not available for comment on Monday morning.
The public prosecutor's office added at around 8.30am that authorities were notified about the discovery of the concerning graffiti and launched an investigation with the Grand Ducal Police.
