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CORINTH, Miss. -- A Mississippi man accused of mailing letters with suspected ricin to national leaders, including President Barack Obama, believed he had uncovered a conspiracy to sell human body parts on the black market and claimed "various parties within the government" were trying to ruin his reputation.

Paul Kevin Curtis, 45, was arrested Wednesday at his home in Corinth, near the Tennessee state line.

RELATED | Curtis describes body-parts conspiracy

Authorities were waiting for definitive tests on intercepted letters that were addressed to President Barack Obama and Sen. Roger Wicker (R-Miss.). Preliminary field tests can often show false positives for ricin.

Ricin is derived from the castor plant that makes castor oil. There is no antidote and it's deadliest when inhaled.

An FBI intelligence bulletin obtain by The Associated Press said the two letters were postmarked Memphis, Tenn.