Initial analysis of two letters sent to the Pentagon tested positive for the deadly poison ricin on Tuesday. The Secret Service is investigating another suspicious envelope sent to President Trump on Monday. It was intercepted before reaching the White House.

Investigators are trying to figure out if one person or a group is behind the packages. Meanwhile the Pentagon's mail delivery center is under quarantine, reports CBS News' Jeff Pegues. One was addressed to Defense Secretary James Mattis and the other to Chief of Naval Operations John Richardson.

The letter addressed to Secretary Mattis, who is currently out of the country, also contained a five by eight card with handwriting on it, but the content has not been made public. Ricin can cause death in 36 to 72 hours and can only be manufactured deliberately.

Bioweapons experts caution field tests are inconclusive and a lab test has to be conducted to confirm that a substance is in fact ricin. Another letter was sent to Sen. Ted Cruz's campaign office in Houston Monday. That tested negative for any suspicious substance.

It is unclear whether the incidents are connected. So far no one has been reported harmed.

