Police in Japan are looking for the person who sent bomb and death threats to numerous schools, forcing hurried closures.
This week, the threats were faxed to colleges and high schools from a Tokyo-registered number.
Police say they have not discovered any explosives in school buildings, and there have been no reports of attacks on students or staff to date.
In Japan, which is renowned for having a low crime rate, bomb threats are uncommon.
The first wave of messages began on Monday, reaching schools and universities across the country. In one prefecture, Saitama, more than 170 schools received bomb threats, according to officials.
Local media reported that one message claimed that over 330 bombs had been set up, while another read: “I planted a major bomb.”
Some reports said the messages demanded ransoms ranging from 300,000 yen (£1,870; $2,320) to 3 million yen.
On Tuesday, messages threatening to kill students and teachers with homemade weapons were sent from the same number to high schools in various prefectures including Osaka, as well as Saitama and Ibaraki near Tokyo.
The threats saw many schools in Japan shutting down as a precaution, though most had re-opened by Thursday.
Fax machines are still commonly used in Japan.