A third envelope containing ricin and addressed to President Donald Trump was intercepted en route to the White House on Monday.

It is believed to be connected to the two similar packages that were sent to the Pentagon.

The first two envelopes that were addressed to Secretary of Defense James Mattis and Chief of Naval Operations John Richardson have tested positive for ricin after being intercepted at the Pentagon, according to officials.

'The Secret Service can confirm receipt of a suspicious envelope addressed to the President on Oct. 1, 2018,' the service said in a statement on Tuesday.

'The envelope was not received at the White House, nor did it ever enter the White House,' the statement added.

'We can confirm that we are working jointly with our law enforcement partners to fully investigate this matter.'

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A third envelope was reported by the Secret Service as being intercepted before arriving at the White House

The first two envelopes triggered alarms during screening at a mail facility which is not inside the main building but is nearby, defense officials said. They have been sent to the FBI for additional testing.

At around the same time, two people were hospitalized after being exposed to a white powder that was contained in mail sent to Sen. Ted Cruz's Houston campaign office. Their injuries are not known. Neither of the victims worked for Cruz's campaign.

It is not known if the two incidents are linked or what the substance sent to Cruz's office was.

All mail sent to the Pentagon via the US Postal Service on Monday was placed under quarantine as a precaution and all workers from Cruz's Houston campaign office were evacuated.

Officials say the first two were addressed to Secretary of Defense James Mattis (pictured left) and Chief of Naval Operations, John Richardson (right)

The envelopes were intercepted in the mailing facility which is not inside the Pentagon (pictured)

CNN cited two unnamed officials who named Mattis and Richardson as the intended targets of the Pentagon mail.

'The Pentagon Force Protection Agency detected a suspicious substance during mail screening at the Pentagon's remote screening facility.

'The envelopes were taken by the FBI this morning for further analysis,' spokesman Col. Rob Manning said in a statement.

All mail that was sent to the Pentagon was placed under quarantine as a precaution.

Ricin exposure can lead to death if the toxin is inhaled directly. It is commonly used in terror plots and is found in castor beans.

It remains unclear what form it was sent to the Pentagon in.

In a statement, the FBI confirmed its investigation.

'On Tuesday, October, 2, 2018, in coordination with the Pentagon Force Protection Agency, 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.

At around the same time as the announcement from the Pentagon was made, two people were hospitalized after being exposed to a white powder at Sen. Ted Cruz's Houston campaign office (pictured)

The FBI is working at the scene in Houston to determine what was in the mail which caused the two people exposed to require hospital treatment. They have so far not said if the two incidents are linked

'As this is ongoing, we will have no further comment,' a spokesman said.

Ricin causes a painful deaths which begin with nausea and vomiting.

Ricin is a naturally occurring toxin which can be extracted from castor beans (pictured are castor oil seeds)

The toxic components then cause internal bleeding and ultimately lead to respiratory failure.

Several terror plots which use the toxin have been foiled in Europe in the past including an intended attack on a London Underground station in 2003 and a separate attack in Germany in June this year.

After initial exposure to ricin, symptoms ordinarily appear in around 10 hours and death can occur within 72 hours.

Since the anthrax attacks which killed five people after the powder was distributed through mail in 2003, the US Government has increased measures to protect workers and officials from exposure to any harmful substances.

They include putting mail through X-Ray machines and scanners which detect vapors and identify anything poisonous or toxic.

In Washington DC, the US Postal Service also samples the air as mail is being sorted before it is sent to government buildings.