Office of Personnel Management says it detected security breach in April, estimates as many as 4 million current and former federal employees may have been affected.
Trip widely seen as making up for not visiting with president last year, and announcing partnership between US, Japan on Let Girls Learn initiative.
Western governments have accused pro-Moscow separatists of using a Russian missile to shoot down the plane, but rebels argue they don't have the capability.
Ex-head of state Khieu Samphan, chief ideologue Nuon Chea face genocide charges at the trial that begins Wednesday in Phnom Penh.
Officials say crudely built, unmanned aircraft was equipped with basic camera containing about 200 aerial photographs, including some taken from directly above presidential Blue House.
The discord was widely attributed to political pressure from China, which would rather deal separately with the five nations with which it has maritime disputes.
