Police in Bulgaria have detained 12 teenagers after a string of hoax bomb alerts forced the closure of several elite high schools in the capital Sofia this week, police said Wednesday. Since Monday, 13 alerts have been called in at schools around Sofia but no explosives were found, the interior ministry said in a statement. On Monday alone, nine schools were shut down, and three remained shut on Tuesday. The most recent scare was at a Jewish school. The families of five of the students questioned by police must pay fines of up to 10,000 euros, a police spokeswoman said. Bomb hoaxes generally take place toward the end of the semester in Bulgaria, with those responsible hoping to dodge final exams.