A U.S. service member was detained in connection with a death of a Japanese national in Okinawa, U.S. officials said, prompting the Japanese government to lodge a protest with the United States.

“We are actively cooperating with Japanese authorities, in response to allegations of serious criminal actions by a Marine assigned to the III Marine Expeditionary Force,” the force said in a statement on social media. “We extend our deepest condolences to the victim’s family and friends.”

U.S. officials did not immediately identify the service member who was arrested, nor did they say whether the person had been charged in connection with the death.

Sanae Takaichi, the Japanese prime minister, described the death in Naha City as am alleged robbery and murder. She confirmed that a U.S. Marine had been detained in connection with the death.

“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.

The U.S. Embassy in Tokyo said it was aware of the incident and was extending “our deepest condolences to the bereaved family of the deceased.”

“At present, an investigation is underway to clarify the facts, and the United States Government continues to cooperate closely with the Government of Japan and local authorities,” the embassy said in a statement.

George Glass, the U.S. ambassador to Japan, said he had spoken on Sunday morning with the Ministry of Foreign Affairs and Okinawa’s governor, adding that his office was providing full cooperation with an investigation into the death.