Citing federal officials it did not identify, NBC News said Friday Shannon Rogers Guess Richardson of New Boston, Texas, was arrested in Arkansas and was to be formally charged Friday.
Richardson is suspected of sending letters containing a form of the poison to Obama, New York Mayor Michael Bloomberg and Mark Glaze, director of Mayors Against Illegal Guns, the report said.
Richardson had told FBI agents her husband sent the letters, but investigators have determined she sent them, NBC reported.
A federal grand jury in Mississippi Monday indicted James Everett Dutschke, 41, of Tupelo on charges he stockpiled ricin and sent threatening letters laced with it to Obama, a senator and a judge.
Dutschke is charged with one count of producing and possessing the toxin for use as a weapon, one count each of threatening Obama, Sen. Roger Wicker, R-Miss., and Lee County Judge Sadie Holland by mail, and one count of falsifying, concealing and covering up material facts to impede the investigation of threatening letters containing ricin to make it appear someone else had sent them.