Police launched an investigation Thursday after an agency under the justice ministry received a fax threatening to blow up all the middle schools in Seoul. 

The Seoul Metropolitan Police Agency said it is tracking down the sender of the message, which was sent to an immigration branch office in Seoul the previous day. 

The message, in Japanese, reads that bombs have already been installed in all middle schools in Seoul and the bombs will go off at 4:33 p.m. Friday. 

Police said similar threats have been made against key domestic agencies via e-mail or fax since August 2023. 

The Seoul Metropolitan Office of Education convened an emergency meeting Thursday and conveyed the latest information to all middle schools in the city. 

The education office urged all schools to boost efforts to control access by outsiders, strengthen patrolling by school security guards, as well as police, and thoroughly review their CCTV monitoring and security systems.