Packages with suspicious substances sent to the Pentagon, White House and Sen. Ted Cruz's office in Texas this week were part of a coordinated effort by a former Navy sailor, officials told Fox News.

Investigators said one of the envelopes, addressed to either Defense Secretary James Mattis or the Navy's top officer, Adm. John Richardson, contained a return address leading them to believe it came from the sailor, whose name has not been disclosed.

A Pentagon spokesperson told Fox News on Wednesday that at least two of the envelopes contained castor seeds -- the substance from which the poison ricin comes from.

Officials say a tip from the White House led the Pentagon to discover the two packages sent to the Pentagon -- and now both are in the hands of the FBI for further testing.

The packages never made their way into the Pentagon, but were flagged in a mail delivery building next door on Monday, a Pentagon spokesperson confirmed to Fox News.

“On Monday, the Pentagon Force Protection Agency detected a suspicious substance during mail screening at the Pentagon's remote screening facility," Col. Rob Manning said in a statement to Fox News. "All USPS mail received at the Pentagon mail screening facility yesterday is currently under quarantine and poses no threat to Pentagon personnel.”

Late Tuesday, the U.S. Secret Service revealed it had received a "suspicious envelope" addressed to President Trump the day before. The agency said the envelope "was not received at the White House, nor did it ever enter the White House."

The Secret Service declined to comment further, except to say it was "working jointly with our law enforcement partners to fully investigate this matter."

Word of the incidents in the Washington area came as authorities said two people were taken to the hospital Tuesday after being exposed to a "white powdery substance" at Sen. Ted Cruz's office in Houston. The Houston Fire Department said on Twitter that the entire 9th floor of the Phoenix Tower was evacuated before all tests came back negative for any hazardous substance. An official for the Republican told Fox News authorities did not "yet know what was the substance."

Ricin is a poison that can take several different forms, but as a purified powder it can be deadly. It's also one of the easiest poisons to manufacture.

Fox News' Travis Fedschun, Cody Derespina, Lillian LeCroy and Greg Norman contributed to this report.