WASHINGTON (AFP) - A letter possibly laced with the deadly poison ricin was found at a United States (US) airbase on Tuesday as a man charged with sending a similar letter to the White House was released on bond, officials said.

The letter discovered at Bolling Air Force Base outside Washington comes a week after three others were intercepted on their way to President Barack Obama, a US senator and a Mississippi official.

The poisoned letters found last week came as the nation was already on edge after a deadly bombing at the Boston Marathon. The Federal Bureau of Investigation has said there is no indication the two cases are linked.

"We've had another incident today, I'm told, at Bolling Air Force Base, the same substance," Senator Harry Reid told reporters.

The letter was discovered Tuesday morning during routine screening of incoming mail, said the Defence Intelligence Agency, which is headquartered at Bolling.

Initial tests "indicate possible biological toxins", the agency said in a statement that did not name the toxin.

"Prudent screening methods and force protection measures were implemented to prevent personnel from being harmed," the agency said, adding that it has "maintained normal operations".

Paul Kevin Curtis, 45, was arrested by the FBI at his home in Corinth, Mississippi last week and charged with threatening the life of the president.

His lawyer insisted on Tuesday that they found the wrong man, noting that the FBI could find "nothing" in Curtis's home or on his computer to link him to the letters.

"We have maintained from the beginning... that Kevin Curtis is absolutely 100 per cent innocent," Ms Christi McCoy told CNN.

"The case has not been dismissed, but obviously we feel better about it than we did this time yesterday." Ms McCoy insisted that Curtis had been framed for the crime, and the FBI was reportedly questioning a 'rival' in nearby Tupelo, Mississippi.

Mr J. Everett Dutschke came out of his home to tell reporters he had nothing to do with the plot shortly before it was surrounded by about 10 law enforcement vehicles, the Northeast Mississippi Daily Journal reported.

A spokesman 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.

A spokesman for the US Marshals Service also did not immediately return a request for comment to confirm that Curtis was released.

The FBI said last week that Curtis was "believed to be responsible for the mailings of the three letters sent through the US Postal Service" that contained "a granular substance that preliminarily tested positive for ricin."

The letters were addressed to Mr Obama, Republican Senator Roger Wicker of Mississippi and a justice of the peace in the same US state, Mr Sadie Holland.

The three notes all referred to "missing Pieces" and were signed "KC", which matched other letters sent to government officials that investigators suspect all trace back to Curtis, according to court documents.

Curtis also wrote about an alleged black market for the illegal sale of human body parts that he thought was being covered up by the government, prosecutors said.

In 2007, Curtis's ex-wife reported to police that her former spouse was extremely delusional and believed the government was spying on him with drones.