Tokyo police have launched an investigation after the South Korean Embassy received a threatening letter alongside what appeared to be a bullet late last month.

Investigative sources say the embassy noticed the delivery of an envelope containing a single sheet of paper and an apparent bullet on Tuesday last week.

The mail was addressed to the former South Korean Ambassador to Japan. The envelope did not bear the sender’s name.

The paper inside said the sender possesses rifles and is plotting to target South Koreans. The person also wrote that South Koreans should leave Japan.

In a separate case on Sunday, police arrested a 67-year-old man who punched and damaged the embassy’s mail box. The man was posting a letter of protest against the South Korean government.

The incidents have taken place amid a serious decline in relations between Tokyo and Seoul.

It remains unclear whether the cases are related to South Korean court rulings on wartime labor or the country’s recent decision to scrap an intelligence-sharing pact with Japan.