A new bomb threat was placed Tuesday morning against a high school in the Bulgarian capital Sofia

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

Earlier Monday, the police informed about only 2 bomb signals, which grew to 3 and then to 5, before reaching 9 total.

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

The Tuesday call has been placed around 11:30 am and was for the 51st Secondary School. Police are still checking the premises. The students have been evacuated and the school is closed for the day as well.

The English, German, French, and Spanish Language High Schools, the Sofia Mathematics High School, the 7th Secondary School, the 31st Secondary School and the Professional School of Fine Mechanics and Optics were among those that received bomb threats Monday on the emergency 112 hotline.

Police have said that most of the anonymous signals came from street phones, but some of them were placed from cell phones.

Police however commented that the case is "without precedent." They note that so far they have no confirmation of speculations that the bomb threats were organized as a prank by a group on the internet and reject any possibility one person made all the calls.

Bulgaria's Penal Code lists such calls as phone terrorism, a crime punishable by 2 years behind bars while minors can be expelled and sent to special disciplinary schools.

From the 9, the Mathematics High School and the Professional School of Fine Mechanics and Optics cancelled classes Tuesday on the decision of their principals.