Police in Japan are investigating death threats against USAmbassador Caroline Kennedy and another diplomat.
The US Embassy in Tokyo reportedly received the threatening phone calls last month from an English speaker.
US Consul General Alfred Magleby, who is based on the island of Okinawa, was also a target of the threats.
Ms Kennedy is the daughter of former US President John F. Kennedy, and became the US ambassador to Japan in November 2013.
Japanese media reports said the phone calls came last month and the caller spoke English.
The caller's motive is not known.
Police in Tokyo and the embassy are refusing to comment.
A police official in Okinawa speaking anonymously to the Associated Press news agency confirmed the investigation that was also being reported on by the Yomiuri Shimbun newspaper.
Mr Magleby is stationed on the southern island of Okinawa, where approximately 25,000 US troops are based.
Reports of the calls surfaced as US First Lady Michelle Obama begins a multiday trip to Japan. She will also visit Cambodia.
Earlier this month, US Ambassador to South Korea Mark Lippert was attacked by a man with a knife and was hospitalised for several days.