The remains of 2,111 unknown soldiers, recovered from battlefields in the vicinity of Washington, were sealed in the vault.
Robert E Lee's former plantation-Arlington, Virginia. Specifically the rose garden beside Lee's mansion.
source-https://www.nps.gov/arho/learn/historyculture/cemetery.htm
After the war, Meigs dispatched crews to scour battlefields for unknown soldiers near Washington. Then he excavated a huge pit at the end of Mrs. Lee's garden, filled it with the remains of 2,111 nameless soldiers and raised a sarcophagus in their honor. He understood that by seeding the garden with prominent Union officers and unknown patriots, he would make it politically difficult to disinter these heroes of the Republic at a later date.
The last autumn of the war produced thousands of new casualties, including Lt. John Rodgers Meigs, one of the quartermaster's four sons. Lieutenant Meigs, 22, was shot on October 3, 1864, while on a scouting mission for Gen. Philip Sheridan in Virginia's Shenandoah Valley. He was returned with solemn honors to Washington, where Lincoln, Stanton and other dignitaries joined his father for the funeral and burial in Georgetown. The loss of his "noble precious son" only deepened Meigs' antipathy toward Robert E. Lee.
"The rebels are all murderers of my son and the sons of hundreds of thousands," Meigs exploded when he learned of Lee's surrender to Grant on April 9, 1865. "Justice seems not satisfied [if] they escape judicial trial & execution... by the government which they have betrayed [&] attacked & whose people loyal & disloyal they have slaughtered."
In the rose garden at Arlington by order of General Montgomery Meigs. The bodies were sealed in a tomb that became the inspiration for the Tomb Of The Unknowns that we see today at Arlington National Cemetery.
Near Arlington House, in what was once part of its famous rose garden, stands a monument dedicated to the unknown soldiers who died in the Civil War. The monument, dedicated in September 1866, stands atop a masonry vault containing the remains of 2,111 soldiers gathered from the fields of Bull Run and the route to the Rappahannock. The remains were found scattered across the battlefields or in trenches and brought here. This monument was the first memorial at Arlington to be dedicated to soldiers who had died in battle, and who later could not be identified. Because in some instances only a few bones or a skull were recovered
Near Arlington House in a part of its famous rose garden. Quartermaster General Meigs ordered this to prevent the Lee family from possibly returning to Arlington.