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Letters suspected of containing deadly poison that were sent to the White House and the Pentagon did not contain a finished form of ricin, a poison made from castor beans that can be deadly, law enforcement officials said Wednesday.

Instead, they contained a primitive form or precursor that is not very hazardous, the officials said.

Asked if other similar letters have been found, one official said that's being looked into, but nothing yet definitive.

"According to our preliminary analysis, the substance was castor seeds, from which ricin is derived," Chief Pentagon Spokesperson Dana White said in a statement about the two letters at the Pentagon mailing center. "The FBI is still investigating."

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The officials also said they now believe a separate letter found yesterday at Sen. Ted Cruz, R-Tx., campaign's office in Houston is not related.

A flurry of suspicious envelopes addressed to high-profile political figures had been received in the past two days.

On Tuesday, federal officials said at least two envelopes, addressed to Secretary of Defense James Mattis and Chief of Naval Operations Adm. John Richardson, at a Department of Defense mail processing center were suspected of containing the deadly toxin.

US Defense Department personnel, wearing protective suits, screen mail at the Pentagon in Washington, DC on October 2, 2018. Mosheh Gains / NBC News

The FBI was leading the investigation in testing those envelopes.

Later Tuesday, the Secret Service said in a statement that it intercepted a "suspicious envelope" addressed to the president.

The mail was intercepted at a location outside of the White House. The Secret Service provided no other details about the envelope.

And two people were hospitalized in Houston after a "white powdery substance" was found in a letter addressed to Cruz's campaign headquarters, according to law enforcement sources. The package was opened in the lobby, resulting in a temporary lockdown, according to Cruz's spokeswoman. No Cruz staffers were hospitalized in the incident, she said.

FBI investigators later said in a post on Twitter that lab results indicated no hazardous substance on that letter.