The White House has been placed on lockdown after a woman threw something over the north fence along Pennsylvania Avenue.
A woman was detained after the incident in Washington DC.
Reporters were not allowed to venture out of the White House press room, and a call to the Secret Service was not immediately returned.
The fire service and a hazardous materials response unit were called to the scene.
It is common for officers to sweep the grounds after such incidents to ensure nothing dangerous was thrown over the fence.
The lockdown ensued shortly after US president Barack Obama returned from delivering a Memorial Day speech at Arlington National Cemetery.