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Everett Dutschke told The Associated Press that the FBI was at his Tupelo, Miss., home Tuesday for a search related to the case. He said his house also was searched last week.

Mr. Dutschke has maintained his innocence and says he doesn’t know anything about the ingredients for ricin.

In an affidavit, investigators said they linked Mr. Curtis to the letters after asking Mr. Wicker’s office staff whether they had received any similar mail.

The office came up with several communications that they said were sent by Mr. Curtis that used the phrase: “this is Kevin Curtis and I approve this message.” The letter to Mr. Wicker this month used a similar phrase: “I am KC and I approve this message.”

The letters were postmarked April 8 in Memphis, Tenn., with no return address.

The FBI said nothing suggests a connection between the letters and the bombings in Boston.

According to the affidavit, Mr. Curtis had been investigated several times by police in Booneville, Miss. Police there said his ex-wife reported that he was “extremely delusional, anti-government, and felt the government was spying on him with drones.”

“He is bipolar, and the only thing I can say is he wasn’t on his medicine,” his ex-wife, Laura Curtis, told The Associated Press last week.

Mr. Wicker said last week that he met the suspect several years ago when Mr. Curtis was hired to perform as an Elvis impersonator at a party for a couple getting married.

The letters were among several pieces of suspicious mail reported by senators last week at Capitol and state offices.

A report Tuesday that another ricin-laced letter had been found at Bolling Air Force Base in Washington turned out to be a false alarm.

This article is based in part on wire service reports.