Paul Kevin Curtis beamed as his lawyer waved the dismissal order at a news conference held shortly after investigators in hazmat suits inspected the home of a man with whom he said he had a longstanding dispute.
"The government was able to basically find another suspect who we believe is the true perpetrator of this heinous crime," defense attorney Christi McCoy told reporters.
Curtis, 45, said the week since his arrest in Mississippi had been a "nightmare for myself and my family," adding he was anxious to "get back to normal."
"I respect President (Barack) Obama, I love my country and would never do anything to pose a threat to him or any other US official," Curtis said.
Three letters laced with ricin were discovered last week as the nation was reeling from a deadly bombing at the Boston Marathon.
The letters were addressed to Obama, Republican Senator Roger Wicker of Mississippi and a justice of the peace in the same US state, Sadie Holland.
The three notes all referred to "missing pieces" and were signed "KC," which matched other letters sent to government officials, according to court documents.
Investigators initially suspected Curtis because he had written publicly about an alleged black market for the illegal sale of human body parts that he thought was being covered up by the government.
But his attorney insisted from the get-go that Curtis was framed and pointed the finger at J. Everett Dutschke, who reportedly also had a long-standing dispute with the Holland family.
Local media published photos of investigators in white hazmat suits exiting the Dutschke home in Tulepo, Mississippi, as it was surrounded by law enforcement vehicles Tuesday.
Dutschke came out of the home before they arrived to tell reporters he had nothing to do with the plot and that he was being questioned by the FBI, the Northeast Mississippi Daily Journal reported.
He was later stopped from reentering his home, photos on the newspaper's website showed.
A spokeswoman for the FBI contacted by AFP declined to comment on the report and referred questions to US prosecutors, who were not immediately available for comment.
The brief order of dismissal filed in federal court Tuesday said the charges against Curtis were dropped "without prejudice" because "the investigation has revealed new information."
Dutschke, 41, is currently out on bond on three charges that he fondled under-age girls, according to the Daily Journal.
He insisted that Curtis was pointing the finger at him in order to dodge his own guilt.
"I guess Kevin got desperate. I feel like he's getting away with the perfect crime," he told the Clarion Ledger newspaper.
"It has made my family incredibly unsafe. It has put a target on us, and it was reckless and irresponsible," he added. "The phone has been ringing off the hook, with calls and hang-ups and all sorts of horrible things."
Dutschke also insisted that he was not a rival to Curtis, as the defense attorney had declared, telling the paper they'd had a minor argument in 2010 but that he hadn't heard from Curtis since.
Meanwhile, a suspicious letter discovered earlier at Bolling Air Force Base outside Washington raised concerns that the ricin scare wasn't over.
While initial tests detected "possible biological toxins" during routine screening, further investigation "found no suspicious packages or letters," said the Defense Intelligence Agency, which is headquartered at Bolling.