US officials said a package containing what is thought to be ricin has been found at a military mail sorting facility in Washington DC.
Senate majority leader Harry Reid told reporters there was an incident at Joint Base Anacostia-Bolling which involved the "same substance" as recent letters sent to US president Barack Obama and a US senator.
A US official said the package was found at a Defence Intelligence Agency (DIA) headquarters mail-sorting facility on the base.
The official added initial tests indicated possible ricin.
The DIA said tests indicated possible biological toxins.