Paul Kevin Curtis of Corinth, Miss., has been arrested in the ricin-tainted letters that were sent to President Barack Obama, U.S. Senator Roger Wicker (R-Miss), and a Lee County, Miss., justice official.
FOX News said the third tainted letter was sent to Lee County Justice Court Judge Sadie Holland.
The two letters addressed to the president and Sen. Wicker were postmarked April 8 and originated in Memphis, Tenn. The third letter to Justice Holland was mailed April 10.
FOX News Channel reporter Mike Levin said the FBI arrested the 45-year-old at his home at around 5:15 p.m. CDT.
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The FBI told FOX News Congressional reporter Chad Pegram that Curtis sent three tainted letters to the president, Sen. Wicker, and a Mississippi justice official.
FOX News has confirmed that the ricin found in the envelopes was "low grade" and "not weaponized."
The suspect was known to frequently write to U.S. lawmakers. Charges have yet to be filed against him.
Officials say the two letters have the same signature that reads, "I am KC and I approve this message."
A 17-minute news conference took place at 9 p.m. Wednesday in Tupelo, Miss., about 50 miles south of Corinth.
The Memphis FBI Joint Terrorism Task Force helped lead the investigation along with the Jackson, Miss., Task Force, local authorities, the U.S. Capitol Police, the U.S. Postal Service, and the U.S. Secret Service.
In a statement from Sen. Wicker's office he thanked the FBI Capitol Police, local police, and added his offices are open for business.
"Gayle and I want to thank the men and women of the FBI and U.S. Capitol Police for their professionalism and decisive action in keeping our family and staff safe from harm," Sen. Wicker said in the media statement. "My offices in Mississippi and Washington remain open for business to all Mississippians. We particularly want to thank the people of Mississippi for their thoughts and prayers during this time."
Authorities still waited for definitive tests on the letters to Obama and Sen. Wicker, which had raised concern Wednesday at a time when many people were jittery after Monday's bombings at the Boston Marathon.
Corinth, Miss., is about 100 miles east of Memphis, Tenn., along the Mississippi-Tennessee state line.