Dutch police have arrested a former student of the British School in the city of Leiden after a threat to carry out a shooting at a school was posted on the internet.
"We have been informed that a former student of the British School in the Netherlands (BSN) has been arrested in connection to threats made against a Dutch school in Leiden," a statement said.
Police in Leiden, just northeast of The Hague, confirmed one person was arrested.
"Someone was arrested in connection with the threat and we're continuing our investigations," a police spokeswoman said.
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More than 20 secondary schools, which are home to students aged 12 to 15, were closed on Monday in response to the threat posted on internet forum 4Chan.
The anonymous internet threat said the writer would shoot his or her teacher "and as many students as I can."
The writer said he or she would be carrying a 9mm Colt Defender gun and a note "which will explain why I did it."
The post on the forum also included a picture of a pistol. Swiss police discovered it during internet checks over the weekend.
The British School said that the arrested former student was expelled in October 2011 "after incidents regarding his behavior and has therefore not attended the school for over a year."
The British School in the Netherlands teaches UK curriculum to students of more than 80 different nationalities.