A letter containing what appeared to be a bullet was sent to the South Korean Embassy in Japan, police said September 3. The incident took place during the period of sharply deteriorating ties between the two countries over wartime history and trade policy.

Police said they are analyzing the object to confirm whether it is a bullet, and to possibly help identify the sender.

The letter was delivered to the embassy in Tokyo (pictured) on August 27 and addressed to former Ambassador Lee Su Hoon, sources close to the matter said. It was sent an anonymously, and contained a threat, indicating that the sender had a rifle and is targeting a South Korean, according to the sources. The police investigation was launched.

Relations between two countries deteriorated swiftly after the South Korean Supreme Court last autumn ordered two Japanese companies to pay damages to South Koreans who it indicated were forced to work in their factories during Japanese colonial rule.

Japan reiterates that the issue of compensation stemming from its colonial rule was settled “finally and completely” in a 1965 bilateral agreement under which Japan provided South Korea with $300 million in grants and $200 million in loans.