The packages, addressed to Defense Secretary James N. Mattis and Chief of Naval Operations Adm. John Richardson, were contained at a mail screening facility outside the main Pentagon complex.
Ricin is a poison that is made from castor beans.
Neither envelope entered the Pentagon.
All mail delivered on Monday is now under quarantine, and "poses no threat to Pentagon personnel", he said.
Brewers, Cubs to meet in NL Central Tiebreaker Monday
ET, ESPN ) to decide the NL Central champion is treated like a regular-season game for statistical purposes. Any tiebreakers would be one-game matchups Monday and would be played at Wrigley Field and Dodger Stadium.
An official said one of the envelops was mailed from inside the United States; it's not clear where the other one came from.
Ricin has often been used in terror plots.
The Houston Fire Department says two people were taken to hospital on Tuesday after apparently being exposed to a "white powdery substance", though it didn't provide details about the two people or report on their conditions.
Two years later, a MS man sent letters containing ricin to President Barack Obama and a Republican senator in an elaborate attempt to frame a rival.
Ricin exposure can lead to death if the toxin is inhaled directly.
Notable Ricin poisonings include the deadly poisoning of Georgi Markov, a Bulgarian writer and journalist who was living in London in 1978.