Officials suspect a former Navy sailor is behind the multiple letters containing suspicious substances sent to the Pentagon, White House and Sen. Ted Cruz Rafael (Ted) Edward CruzTrump argues full Supreme Court needed to settle potential election disputes Press: Notorious RBG vs Notorious GOP The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE's (R-Texas) office in his home state this week, Fox News reported Wednesday.

Investigators told the media outlet that one of the envelopes sent to the Pentagon had a return address leading back to the sailor, whose name has not been revealed.

Multiple letters mailed to Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE and Chief of Naval Operations John Richardson on Monday tested positive for ricin, a toxic substance that is easy to produce and has been used in terror plots.

The Associated Press quoted a Pentagon spokeswoman who said the substance turned out to be castor seeds, from which ricin is derived.

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The White House was also sent a suspicious envelope addressed to President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE, which the U.S. Secret Service intercepted on Monday.

A White House tip reportedly led officials to discover the envelopes sent to the Pentagon.

The Pentagon Force Protection Agency then detected a suspicious substance during mail screening at the building's remote screening facility, according to the Defense Department.

The envelopes were taken by the FBI for further analysis.

"All [United States Postal Service] mail received at the Pentagon mail screening facility yesterday is currently under quarantine and poses no threat to Pentagon personnel,” spokesman Col. Rob Manning said in a statement.

Also on Tuesday, two people were taken to the hospital after they were exposed to a piece of mail sent to Cruz's Houston-based campaign headquarters containing a white powdery substance.

Authorities are still testing the substance.

Updated at 1:57 p.m.