Authorities dropped charges Tuesday against the man they had accused of sending ricin-laced letters to the White House, a U.S. senator and a county judge as the FBI appeared to turn its attention toward his longtime antagonist in a small-town Mississippi feud.

A federal magistrate judge directed that charges against Paul Kevin Curtis, 45, be dismissed because “the ongoing investigation has revealed new information,” according to his written order. The charges were dropped without prejudice, meaning that they could be reinstated.

But Tuesday afternoon, the FBI was searching the Tupelo, Miss., home of James Everette Dutschke in connection with the ricin case. Dutschke said he is innocent and does not know anything about the ingredients for ricin.

It was clear that investigators are dealing with two men with histories of erratic behavior who nearly came to blows as they quarreled, according to the account that Curtis gave in a colorful, rambling news conference outside the federal courthouse in Oxford, Miss., late Tuesday afternoon.

The FBI in Mississippi and Washington, as well as the U.S. attorney’s office in Oxford, refused repeated requests to explain their about-face on Curtis and declined to say whether they are focusing on Dutschke as the man who might have sent three letters containing the deadly poison.

The case bears some resemblance to the FBI’s pursuit of scientist Steven J. Hatfill, who was investigated for nearly five years in connection with deadly anthrax mailings in 2001. The former bioweapons researcher was not formally cleared by prosecutors until 2008, after the death of another man, bacteriologist Bruce E. Ivins, who had become the leading suspect before he succumbed to a drug overdose.

Curtis is known for detailed Internet diatribes, his long-held conspiracy theory about underground trafficking in human body parts — which he has turned into a novel-in-progress called “Missing Pieces” — and his work as an Elvis impersonator. The Corinth, Miss., man has been arrested four times since 2000 on charges that include cyber-harassment.

Dutschke, 41, a martial arts instructor, was charged in January with two counts of child molestation, according to the Lee County Courier, and later released on bail. He was previously convicted of indecent exposure, according to numerous media accounts. He could not be reached at his home or martial arts studio on Tuesday.

At the news conference, Curtis said he and Dutschke had a falling-out and described e-mail exchanges between them that culminated in his challenge to meet Dutschke for a fight that never occurred. “Where his anger and hate started from, I don’t know,” Curtis said of Dutschke.

Dutschke acknowledged his conflict with Curtis and told the Associated Press that their last contact was in 2010, when Dutschke threatened to sue Curtis for saying that he was a member of Mensa, a group for people with high IQs. Dutschke ran unsuccessfully for the Mississippi House of Representatives in 2007.

He told the Clarion-Ledger of Jackson, Miss., in a telephone interview Tuesday that he didn’t know why his name was brought into the case. “I guess Kevin got desperate. I feel like he’s getting away with the perfect crime,” Dutschke said.

He said he feels as if he is the target of a defense tactic. “I don’t know anything about this. Where are the allegations coming from?” he asked. “Who made the allegations? The defense attorney for the accused.”

Curtis’s release came a day after an FBI agent told a court that a search of his home turned up no ricin and that investigators did not find any evidence that he was making it. No other physical evidence tying Curtis to the ricin mailings was presented in two days of federal court hearings, and a third day of hearings was canceled Tuesday morning without explanation.

Curtis’s attorney, Christi McCoy, has maintained that her client is innocent. She said Tuesday that “it took a lot of planning, determination and patience” to carry out the ricin attacks.

“That is so not Kevin, to spend hours focused on making ricin,” she said.

Calls to Curtis’s father, brother and ex-wife were not returned Tuesday afternoon.

Curtis was arrested at his home last Wednesday and charged with sending letters containing ricin to President Obama, Sen. Roger Wicker (R-Miss.) and a county judge in Mississippi. The first letter, to Wicker, was discovered April 15.

According to an FBI affidavit supporting the charges, Curtis allegedly mailed three identical letters on yellow paper laced with a poison thought to be ricin. The letters alluded to a long-held conspiracy theory about the trafficking in human body parts that Curtis had sought to expose.

White House spokesman Jay Carney, when asked about the ricin case, referred questions to the FBI.

Ricin is made from castor beans, and authorities have long worried about its use by terrorists and others. But FBI agents testified this week that they found no castor beans at Curtis’s house nor any information on his computer that he was researching the poison.

Earlier Tuesday, Senate Majority Leader Harry M. Reid (D-Nev.) told reporters that there was another “alleged ricin incident” at Joint Base Anacostia-Bolling in Southeast Washington, but he could not provide additional details. Jacqueline Maguire, an FBI spokeswoman, said the bureau was investigating a suspicious letter at the Air Force base but had no further information.

But the Defense Intelligence Agency released a statement late Tuesday saying that no suspicious packages or letters had been found.

“Today, DIA’s mail screening equipment alerted officials to the possible presence of a potentially harmful substance,” Lt. Col. Thomas Veale, a DIA spokesman, said in the statement. “After thorough on-scene investigation, no suspicious packages or letters were located. The FBI took samples and will conduct further testing off-site.”

Asked at the news conference what his immediate plans were, Curtis said: “Find my dog, Moo Cow. Moo Cow got away when Homeland Security swarmed in on me when I went to check my mail. I haven’t heard anything. I’m just really worried about her. . . . My brother has found Moo Cow! . . . She’s an amazing dog. I want to get to her. I want to get to my children. I haven’t seen any of my children in a week.”

Ed O’Keefe, Aaron C. Davis and Julie Tate contributed to this report.