AFP reports citing officials.
Shannon Richardson, also known as Shannon Guess and Shannon Rogers, was charged with mailing a threatening communication to the president of the United States, according to the US Attorney's Office for the Eastern District of Texas.
The 35-year-old was arrested in Mount Pleasant, Texas and appeared before US Magistrate Judge Caroline Craven in Texarkana. She faces up to 10 years in federal prison if convicted.
"According to the arrest affidavit, on May 20, 2013, Richardson is alleged to have actually mailed three letters containing the toxin ricin," the US Attorney's Office said in a statement.
The activist who received a letter is Mark Glaze, who heads the Mayors Against Illegal Guns group that Bloomberg helped found.
Bloomberg has said that the letters "obviously referred to our anti-gun efforts."
A copy of the letter sent to Bloomberg posted online showed a stained, type-written document that contained numerous spelling errors.
"You will have to kill me and my family before you get my guns. Anyone wants to come to my house will get shot in the face," it read.
"The right to bear arms is my constitutional God given right and I will excersice that right til the day I die. whats in this letter is nothing compared to what ive got planned for you."
Reports said that Richardson initially had told the FBI that her husband Nathan Richardson may have sent the letters.
She claimed to have found castor beans -- used to make ricin -- and ricin-related Internet searches on their computer at home.