Authorities found a bottle appearing to contain a liquid in a basement ladies' rest room, along with the note.
None of the objects was found to be a threat, said Wayne Benson, a batallion chief with the District of Columbia fire department.
The memorial was reopened less than three hours later, shortly after 3 p.m.
No injuries were reported.
Firefighters, U.S. Park Police and the FBI investigated after authorities found a Gatorade bottle and a travelers' coffee mug in the rest room, Benson said. Several pieces of paper inside a plastic case also were found.
During the closure, hazardous materials crews in protective suits were seen working at the memorial.