Charges have been dropped against a US man accused of sending ricin-laced letters to president Barack Obama, while authorities searched another man's home in connection with the case.
The surprising move was announced in a brief document filed in federal court in the state of Mississippi, hours after Paul Curtis was released from custody.
The charges were dismissed without prejudice, which means they could be reinstated if prosecutors choose.
Lawyers for Curtis have suggested he was framed, and an FBI agent testified in court this week that no evidence of ricin was found in searches of his home. At a news conference, they declined to discuss whether they were told what new information the government had uncovered.
In Tupelo, numerous law enforcement officers converged on the home of another Mississippi man, including some in hazmat suits. Everett Dutschke said the FBI was at his home for the search connected to the mailing of poisoned letters to Mr Obama, Republican Senator Roger Wicker and a state judge. Dutschke said his house was also searched last week.
Dutschke has maintained his innocence and says he does not know anything about the ingredients for ricin. He said agents asked him about Curtis, whether Dutschke would take a lie detector test and if he had ever bought castor beans, which can be used to make the potent poison. "I'm a patriotic American. I don't have any grudges against anybody. I did not send the letters," said Dutschke, who has not been arrested or charged.
Meanwhile, an investigation into a possible suspicious letter at a security facility in Washington DC turned out to be a false alarm, a defence official said.
While mail screening equipment alerted officials to the possible presence of a toxin such as ricin, no suspicious letters or packages were found at Joint Base Anacostia-Bolling, according to Lieutenant Colonel Thomas Veale, spokesman for the Defence Intelligence Agency.
Lt Gen Veale said the FBI had taken samples and will do further testing.
Earlier Senate majority leader Harry Reid told reporters there had been an incident at the Bolling base that involved the "same substance" as recent letters sent to president Barack Obama and a US senator. Other officials said initial testing at the DIA mail-sorting facility suggested it was ricin.