(CNN) An envelope addressed to President Donald Trump contained a substance suspected to be ricin and appeared to be connected to similar envelopes sent to the Pentagon, a law enforcement source told CNN.

Two pieces of mail delivered to the Pentagon mail facility on Monday have initially tested positive for ricin, according to a US defense official.

When asked about the letters sent to the Pentagon, a Secret Service spokesman told CNN that "the Secret Service can confirm receipt of a suspicious envelope addressed to the President on Oct. 1, 2018."

"The envelope was not received at the White House, nor did it ever enter the White House," the spokesman continued.

A source familiar with the ongoing joint federal investigation said that based on preliminary investigative activity, the White House and Pentagon letters were believed to be connected and the substance in question was a very crude castor bean concoction that authorities were not technically calling "ricin" until further testing.

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