Japanese police said they arrested a man on Thursday accused of making bomb threats against the U.S. Embassy in Tokyo, and media reports said he is also a possible suspect in recent death threats against Ambassador Caroline Kennedy.

Tokyo police arrested Mitsuyoshi Kamiya, a resident of the southern island of Okinawa, for making the bomb threats from a pay phone in downtown Okinawa, a police spokesman said. He said the suspect also threatened to bomb Camp Schwab, a U.S. military base on the island.

Kamiya admitted to police that he made the bomb threats, but his motive was not immediately known, the police spokesman said. He did not provide any other details about Kamiya.

Public broadcaster NHK said police also suspect Kamiya made phone calls threatening to kill Kennedy and the U.S. consul general on Okinawa.

The island is home to about half of the 50,000 American troops in Japan, and residents have frequently complained about crime, noise and other issues related to the U.S. bases. The U.S. government wants to relocate one base, the Marine Corps Futenma air station, to another area of Okinawa, but many people want it moved completely off the island.

In three phone calls in March, Kamiya used threatening phrases in English such as "Bomb Camp Schwab" and "Bomb the embassy" which forced the embassy to increase its security, the police spokesman said on condition of anonymity because he was not authorized to speak to journalists who are not members of the police press club, which is largely limited to Japanese media.

The mass-circulation Yomiuri newspaper said police had been secretly following the suspect. It said the embassy had received about 30 threatening calls since last month, including those threatening to kill Kennedy and the consul general.

On Wednesday, State Department spokeswoman Jen Psaki said the U.S. government takes threats to American diplomats seriously, and that it would not comment on specific details of any threats.

Kennedy, the daughter of the late President John F. Kennedy, arrived in Tokyo in November 2013 as the first woman to serve as U.S. ambassador to Japan.

She visited Okinawa in February last year in an attempt to win support for the base relocation plan, and pledged that Washington would do its best to reduce the burden of its heavy troop presence there.

Earlier this month, the U.S. ambassador to South Korea, Mark Lippert, was slashed in the face and wrist by an anti-U.S. activist in Seoul and was hospitalized for several days.