A United States service member is in custody on suspicion of killing a Japanese national on the island of Okinawa.

Lance Corporal Devin Jacob Ballard, 20, is suspected of killing Anna Yagi, 39, in a hotel in Naha, the capital of the southernmost prefecture, on Saturday and taking her wallet and backpack, ​local police said. Ballard denied the allegations, police said.

The Marine was stationed at Futenma Air Station on Okinawa Island, police said, adding that he was arrested at 1 a.m. local time early Sunday.

“An investigation is underway to determine the facts of the case, the United States is communicating and cooperating with the Government of Japan and local authorities,” the U.S. Embassy in Tokyo said Sunday in a post on X.

NBC News has contacted the Pentagon for further comment.

Japanese prime minister Sanae Takaichi described the death as an alleged robbery and murder.

“The occurrence of such an extremely brutal and heinous incident is a matter of profound regret, and as the Government of Japan, we have lodged a strong protest with the U.S. side,” she said on social media.

Japan’s Ministry of Defense said it was “deeply outraged by the extremely brutal and heinous crime committed by a U.S. Marine.”

It said on X that it had made “a firm demand for stricter discipline, thorough measures to prevent recurrence.”

The III Marine Expeditionary Force, a combat force of the U.S. Marine Corps headquartered in Okinawa, said it was cooperating with Japanese authorities in response to “allegations of serious criminal actions by a Marine.”

“The Marine Corps takes these allegations very seriously and expects every Marine to adhere to the highest standards of behavior,” it posted on X. “The Marine is in custody of local authorities while investigations proceed.”

The U.S. Ambassador to Japan, George Glass, said he had spoken with Japan’s Ministry of Foreign Affairs and Okinawa Governor Genta Koja, “pledging full cooperation as the investigation continues.”

There are 18,000 U.S. Marines based in Japan, the majority of them stationed in Okinawa, just 60 miles from Taiwan at its closest point. The U.S. held the island from the end of World War 2 until 1972, and the presence of U.S. military has often stirred resentment among residents.

The 1995 abduction and rape of a 12-year-old girl by three U.S. servicemen sparked mass protests, as tens of thousands of people took to the streets, forcing Washington and Tokyo to discuss ways to reduce the U.S. military presence on the island.