Federal authorities are investigating mail sent to senior Pentagon officials that is believed to contain the poison ricin, officials said Tuesday.

The Pentagon Force Protection Agency on Monday detected a suspicious substance on two envelopes at a mail facility on Pentagon grounds in northern Virginia, the officials said. Initial tests indicated that the envelopes, addressed to Defense Secretary Jim Mattis and Chief of Naval Operations Adm. John Richardson, contained the toxic material.

Col. Robert Manning, a Pentagon spokesman, said the FBI had taken 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 in the incident at the mail facility, which is not part of the main Pentagon building.

A FBI spokesman said the envelopes were “currently undergoing further testing.”

According to the Centers for Disease Control and Prevention (CDC), ricin is a naturally occurring byproduct of castor oil and can be made in the form of a pellet, powder or mist. It is usually dangerous only when used in an intentional attack, the CDC said on its website.

“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.

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The Washington Post’s Matt Zapotosky contributed to this report.