38 USC 2409 - Memorial areas in Arlington National Cemetery

(a) The Secretary of the Army may set aside, when available, a suitable area or areas in Arlington National Cemetery, Virginia, to honor the memory of members of the Armed Forces and veterans
(1) who are missing in action;
(2) whose remains have not been recovered or identified;
(3) whose remains were buried at sea, whether by the members or veterans own choice or otherwise;

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(4) whose remains were donated to science; or
(5) whose remains were cremated and whose ashes were scattered without interment of any portion of the ashes.
(b) Under regulations prescribed by the Secretary of the Army, appropriate memorials or markers may be erected in Arlington National Cemetery to honor the memory of those individuals, or group of individuals, referred to in subsection (a) of this section.