A bomb threat was sent via email to the South Korean embassy in Tokyo last week, the embassy said on Monday. The message threatened to blow up the building ahead of an anniversary celebrating the end of Japanese rule of the Korean Peninsula, according to a media report.

Japanese authorities ramped up security in the area after the threat, which was sent by someone using the name “Soma Wataru,” South Korea’s Yonhap news agency reported.

The message, written in Korean, is said to have read, “I am Japanese. I warn you about the explosion in advance.” But the sender did not specify a date or time, the report said.

The embassy is investigating the possibility of the threat being a hoax, but it has also asked Japanese police to provide stepped up security for almost a month, the report said.

On Wednesday, an email threatening to blow up the National Museum of Korea, the Japanese Embassy, the N Seoul Tower and a Japanese school was reported to the police, Yonhap reported. No explosives were found by authorities.

National Liberation Day, which celebrates liberation from Japanese rule, falls on Aug. 15.