The FBI is leading the investigation, the Examiner reported.
There are no reports of anyone falling ill because of the suspected substance, the Examiner reported.
The pieces of mail were intended for Secretary of Defense James Mattis and Chief of Naval Operations Admiral John Richardson, CNN reported.
More than 20,000 people work in the Pentagon, according to CNN.
Ricin is one of the easiest poisons to make and can take several forms, according to Fox News. It is found in castor beans and the waste from the process of making castor oil is where ricin comes from, according to the Centers for Disease Control and Prevention.