Traces of ricin were found in the martial arts studio of a US man charged in a bizarre plot to send poisoned letters to President Barack Obama and other public officials, the FBI said Tuesday.

The letters were intercepted as the nation was still reeling from the Boston Marathon bombings, but the nerve-wracking plot took a strange turn last week when it turned out that the motivation was likely revenge, not terrorism.

James Everett Dutschke, 41, is accused of trying to frame a fellow Mississippi man with whom he'd had what the FBI described as a "contentious relationship." At first, the alleged plan seemed to have worked.

Elvis impersonator Paul Kevin Curtis, 45, was arrested after officials found that the ricin-laced letters contained the title of a book he was writing about an alleged black market for human body parts.

The letters were also littered with phrases Curtis used on his Facebook page and in correspondence with the Mississippi senator who was also targeted.

Curtis immediately pointed the finger at his rival and the FBI turned its attention to Dutschke, who also had a long-standing dispute with a Mississippi judge who was among the three targets of the poisoned letters.

Dutschke apparently got nervous after defense attorneys blamed him publically for the plot and an FBI agent testified that no ricin was found in Curtis's home.

But he didn't notice the FBI agents who were following him as he went to his former Taekwondo dojo on April 22 to pick up a coffee grinder -- which can be used to extract ricin from castor beans -- a dust mask and latex gloves.

He was spotted dumping them out the window of his van into a public garbage bin about 100 yards away, charging papers said.

The FBI picked them out of the trash and also collected the contents of the bin outside Dutschke's house. Many of the items tested positive for ricin, as did the drain traps at the dojo, the charging papers said.

Dutschke also tried to wipe his computer clean of any traces of his research by reinstalling the operating system on April 22.

But the computer had already been searched by state police after he was arrested in January on child molestation charges.

So the FBI was able to detect that he'd downloaded two documents describing the safe handling and detection of ricin on December 31.

They also found records that Dutschke ordered 100 red castor beans off eBay in November and December.

"I understand that the number of castor beans ordered is more than sufficient to extract the quantity of ricin found in the three letters," Special Agent Stephen Thomason wrote.

Dutschke faces up to life in prison if convicted of "knowingly developing, producing, stockpiling, transferring, acquiring, retaining and possessing a biological agent, toxin and delivery system, for use as a weapon."