Col. Robert Manning, a Pentagon spokesman, said the FBI took possession of the packages on Tuesday morning and would conduct additional tests.
“All [U.S. Postal Service] mail received at the Pentagon mail screening facility yesterday is currently under quarantine and poses no threat to Pentagon personnel,” Manning said in a statement.
No one was injured at the mail facility, which is not part of the main Pentagon building.
An FBI spokesman said the envelopes were “currently undergoing further testing.”
Later, on Tuesday, the Secret Service said that another piece of mail, addressed to Trump, had also been identified on Monday as suspicious.
“We can confirm that we are working jointly with our law enforcement partners to fully investigate this matter,” the Secret Service said in a statement. It said the envelope was not received at the White House.
CNN reported that the envelope intended for Trump was also believed to contain ricin and that officials believed the two incidents may be linked.
According to the Centers for Disease Control and Prevention, ricin is a naturally occurring byproduct when castor oil is made from castor beans and can be in the form of a pellet, powder or mist. It is usually dangerous only when used in an intentional attack, the CDC website says.
“We are maintaining our vigilance for all packages coming into not just the Pentagon but facilities worldwide,” a defense official said, speaking on the condition of anonymity to describe precautions taken by the military.
Matt Zapotosky contributed to this report.