9 high schools in Bulgarian capital Sofia, some of them among the most prestigious, have received bomb threats Monday.

Earlier Monday afternoon, the police had informed about only 5 bomb signals, but the number quickly grew to 9 soon after.

No bombs have been discovered at any of the schools, but classes have been suspended for the day.

Among the institutions in question are the English, German, French, and Spanish Language High Schools, the Sofia Mathematics High School, the 31st Secondary School and the Professional School of Fine Mechanics and Optics.

Police have said that most of the anonymous signals came from street phones, but some of them have come from mobile devices.

They have said that this far they have no confirmation of speculations that the bomb threats were organized as a prank by a group on the internet.

Police however commented that the case is "without precedent."