By Agence France-Presse
Defense Department spokesman Chris Sherwood said at least two suspicious packages were intercepted at a nearby screening center on Monday.
"As part of the screening process, (authorities) recognized some suspicious packages," Sherwood said, noting they were "suspected to be ricin."
He stressed that authorities were still waiting for confirmation that the packages contained ricin. Pentagon police referred the matter to the FBI for investigation.
A defense official told AFP the packages were addressed to Defense Secretary Jim Mattis and Navy head Admiral John Richardson.
"FBI Special Agents took possession of two suspicious envelopes that had been screened at the Pentagon mail facility. Those envelopes are currently undergoing further testing," the FBI said in a statement.
Pentagon spokesman Colonel Rob Manning said all mail received at the Pentagon's screening facility Monday was "placed under quarantine and poses no threat to Pentagon personnel."
Pentagon authorities showed reporters the secure area where mail is received.
Defense Department workers in white protective suits screen every piece of post that comes in, some 500,000 items per year, officials said.
Produced by processing castor beans, ricin is lethal in minute doses if swallowed, inhaled or injected and 6,000 times more potent than cyanide, with no known antidote.