The youth arrested on Sunday in connection with a threat to shoot a teacher and pupils at a school in Leiden has been released without charge and is no longer a suspect, police said on Tuesday.

The youth was picked up after foreign police alerted the Dutch authorities to an English language threat on website 4chan. The report led to all schools in Leiden being closed on Monday.

Leiden mayor Henri Lenferink said the fact the youth, who is said to be under 18, has been released means there is still some measure of a threat. Police are still patrolling the city’s secondary schools, the mayor said.

Proxy server

‘We now know more about the person who put up the message. We seriously consider it came from Costa Rica,’ Lenferink told Radio 1 news.

However, a police spokesman said there is no certainty the threat was made from Costa Rica and it may have been made via a proxy server to hide the origins.

There is no word on why the boy was arrested in the first place or why the fact he was a former pupil at the British School in Voorschoten was made public. The youth was expelled by the school in 2011.