Several mud-caked headstones line the banks of a small stream at Arlington National Cemetery, the country's most venerated burial ground. Farther upstream in a wooded area, a few others lie submerged with the rocks that line the stream bed.

On Wednesday, after The Washington Post alerted the cemetery to their presence, officials there said they were shocked to find the gravestones lying in the muck near a maintenance yard. Already under fire in recent days for more than 200 unmarked or misidentified graves and a chaotic and dysfunctional management system, cemetery officials vowed to investigate the headstones along the stream and take "immediate corrective action," said Kaitlin Horst, a cemetery spokeswoman.

Officials said they do not know how the stones got there, whom they belong to, or how old they are. Horst could say only that "they appear to be decades old."...