A man charged with sending a poison-laced letter to the White House was released on bond Tuesday as a letter sent to a US airbase raised similar fears before it was found to be harmless.

The United States remains on edge after a deadly bombing last week at the Boston Marathon, and the suspicious letter discovered at Bolling Air Force Base outside Washington raised concerns.

Senator Harry Reid told reporters the suspicious letter contained the same deadly substance -- ricin -- as three others that were intercepted on their way to President Barack Obama, a US senator and a Mississippi official last week.

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.

Meanwhile, the case against Paul Kevin Curtis, 45 -- who was arrested by the FBI at his home in Corinth, Mississippi last week and charged with threatening the life of the president -- appeared to be falling apart.

His lawyer insisted Tuesday that the FBI found the wrong man, noting that they 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 percent innocent," Christi McCoy told CNN.

"The case has not been dismissed, but obviously we feel better about it than we did this time yesterday."

McCoy insisted that Curtis had been framed for the crime, and the FBI was reportedly questioning a 'rival' in nearby Tupelo, Mississippi.

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 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.

A spokesman for the US Marshals Service 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 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 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.