A Mississippi man accused of sending ricin-laced letters to US President Barack Obama and a senator has been released from jail with his charges dismissed.

The release of Paul Kevin Curtis came as investigators on Tuesday inspected the home of another 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," defence attorney Christi McCoy told reporters.

In a court order dismissing the charges, prosecutors said the "ongoing investigation has revealed new information" without providing any addition detail.

Curtis, 45, told a news conference afterwards that he respected Obama and would never harm a public servant. "I love this country," he said.

Suspect 'framed'

McCoytold CNN she believed her client had been framed.

"I do believe that someone who was familiar and is familiar with Kevin just simply took his personal information and did this to him," McCoy said. "It is absolutely horrific that someone would do this."

Curtis was arrested last Wednesday at his home in Corinth, Mississippi. He was charged with mailing letters to Obama, US Senator Roger Wicker of Mississippi and a state judge containing a substance that preliminarily tested positive for ricin, a highly-lethal poison made from castor beans.

The letters were intercepted by authorities before they reached their destinations.

Over the weekend, investigators searched Curtis' home, his vehicle and his ex-wife's home, but failed to find any incriminating evidence, McCoy told the Northeast Mississippi Daily Journal.

The poison scare put Washington on edge during the same week the Boston Marathon bombings occurred.

Mental illness

Curtis, known in Mississippi as an Elvis impersonator, was held in the Lafayette County Detention Center prior to his release. He was charged with threatening to harm Obama and using the mail to make other threats.

In a statement last week, his family said they had not been shown any evidence of the charges against him, but added that Curtis suffers from a long history of mental illness.

Typewritten on yellow paper, the three letters contained the same eight-line message, according to an affidavit from the FBI and the Secret Service filed in court.

"Maybe I have your attention now / Even if that means someone must die," the letters read in part, according to the affidavit. The letters ended: "I am KC and I approve this message."

The initials "KC" led law enforcement officials to ask Wicker's staff if they were aware of any constituents with those initials, and the focus of the investigation then turned to Curtis, the affidavit said.

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.