A US sailor in Japan is being held on suspicion of involvement in a local woman's death, the US Navy has said.
The USS Kitty Hawk is based in Yokosuka
The sailor is being held in US custody near where the incident took place, in Yokosuka - where the USS Kitty Hawk aircraft carrier is based.
Yoshie Sato died on Tuesday, after being found beaten and unconscious.
Her death comes at a time when the US is trying to win local support to station a new nuclear-powered aircraft carrier at Yokosuka.
Analysts say this recent incident could complicate sensitive negotiations between Washington and Tokyo over the redeployment of US forces in Japan.
Incidents involving US troops and local Japanese are particularly sensitive because of a 1995 case when three US servicemen raped a schoolgirl.
Police believe Yoshie Sato was attacked during a robbery, according to local news reports.
The sailor, who has not been identified, is being held pending an investigation into the killing, the US Navy said in a statement on Friday.
Japanese media reported on Thursday that the sailor had already confessed to the killing, but Commander John Wallach, director of public affairs for the US Naval Forces in Japan, told Reuters news agency he could not confirm or deny those reports.
The US Navy statement said it had ordered a "period of reflection" for all its personnel, imposing a temporary curfew requiring them to be back on base by midnight.
"The entire navy community in Japan is deeply saddened by this incident," the statement said.
Rear Admiral James Kelly, Commander of the US Naval Forces in Japan, offered his "deep regret and sadness" over the death.
The US statement said the navy was co-operating closely with the Japanese authorities, but it remains unclear whether the sailor will be handed over to Japanese custody.
A US-Japan pact governing the conduct of military personnel in Japan does not require such a transfer until suspects are charged.
But after the 1995 rape of a Japanese schoolgirl by three US servicemen on the island of Okinawa, Washington agreed to seriously consider handing over suspects accused of serious crimes.
About 50,000 US troops are stationed in Japan under a joint security pact, but Tokyo and Washington agreed late last year to remove 7,000 marines and shift other remaining troops within Japan.