DoD said in a tweet that it handed over the suspicious mail to FBI for further analysis.
The Wall Street Journal reported that the mail distribution facility performed a test on the letters that were addressed to Defense Secretary James Mattis and Adm. John Richardson, chief of naval operations, and found that the envelopes tested positive for the poisonous substance derived from the castor oil plant.
Army Col. Rob Manning, a spokesman for DoD, said all letters that arrived Monday at the Pentagons mail screening center were placed under quarantine and do not pose a threat to DoD personnel.
FBI said it would run a second test on the letters to validate the preliminary results.
The Secret Service also identified a suspicious mail addressed to President Donald Trump on Monday, according to the report.
The envelope was not received at the White House, nor did it ever enter the White House, the Secret Service said in a statement.