Armed police were deployed and schools closed in the western Dutch city of Leiden on Monday after an apparent student threat to "shoot my Dutch teacher and as many students as I can".
A decision was taken late Sunday night to close secondary schools to thousands of pupils following a tip-off by Swiss police concerning a threat made on the Internet over the weekend.
"I will shoot my Dutch teacher and as many students as I can," said the anonymous message in English on the US-based online forum 4chan.org.
"It's at a school in the Dutch city of Leiden" it said, chillingly adding "for more proof, I will be using a 9mm Colt Defender."
"Oh, and I'm using a proxy, the police is not gonna find me before tomorrow," the message ended.
"There are police guarding every secondary school in Leiden this morning," the Dutch national broadcaster NOS reported on Monday.
Police declined to give numbers or further details when contacted by AFP, referring all queries to the city council, which was not immediately available for comment.
In a statement late Sunday, law officials said they did not yet know against which school the threat had been made and warned parents not to send their children to class.
An AFP correspondent said that besides the cordoned-off schools and the police presence, life in the city appeared to be continuing normally.
Leiden mayor Henri Lenferink told the NOS broadcaster "we received a report via the police in Zurich, who saw the posting on the Internet and gave it through to the police in Leiden."
"They tried to establish who placed the text but that was not immediately successful. There has also been and still is, contact with the FBI in the United States" to see if the person can be traced, Lenferink said.
He said after running out of time, the decision was taken to close the schools because "if you can't find the perpetrator or know exactly which school is being targeted, only that it's a school in Leiden... you have to do something."
Lenferink added that "of course its possible that it's a morbid joke."
"On the one hand you don't want to overreact, on the other nobody wants to take the risk -- that's the difficult dilemma we were facing yesterday," he said.
Secondary schools in the Netherlands cover the 12-to-18 age group and there are at least 22 such schools in Leiden -- some of them international. The city, which also hosts the country's oldest university, lies just northeast of The Hague.
Dutch police in 2009 arrested an 18-year-old man for posting threats on 4chan.org.
The man, who threatened a shooting at a school in the southern city of Breda, later said he did it as a joke.
The 4chan.org website describes itself as a "simple image-based bulletin board," covering a variety of topics including video games, sports, Japanese anime and pornography.
In April 2011, a man armed with a semi-automatic rifle killed six people when he opened fire on shoppers at a mall in the western town of Alphen aan de Rijn.
Gunman Tristan van der Vlis, 24, then committed suicide, in what was the worst mass shooting in recent times in the Netherlands.
In 1999, four students and a teacher were wounded when a student fired 10 shots in a computer room at a school in the southern Dutch town of Veghel.