Neither men were exposed to the toxin, since all mail to the Pentagon is routed through a mail screening facility that is nearby but separate from the building.
The other letter was addressed to the US Navy's top officer, Admiral John Richardson, one official said.
Officials say the packages were delivered on Monday.
The FBI said that the substance was undergoing further testing.
Meanwhile, all US mail at the Pentagon is being held in quarantine while the tests are taken, and there is no health risk to Pentagon workers, the defense department said in the statement.
All mail received Monday at the Pentagon facility was quarantined, according to the statement.
"The envelope was not received at the White House, nor did it ever enter the White House", the spokesman said.
Ricin is made from the leftover by-products of castor oil. They were turned over to the Federal Bureau of Investigation.
Pentagon spokesman Chris Sherwood said the envelopes had been found on Monday (local time). Both envelopes were tested by the force protection agency and came out positive initially for ricin. "As a matter of practice, the Secret Service does not comment regarding matters of protective intelligence", the statement said.
Ricin is a poison made from castor beans. Centers for Disease Control and Prevention, ricin "can be in the form of a powder, a mist, or a pellet, or it can be dissolved in water or weak acid".
Highly toxic, it can cause nausea, vomiting, internal bleeding and difficulty breathing, leading to death from organ failure or the collapse of the circulatory system.