The FBI is now investigating a second contaminated envelope sent to a mail processing facility in Washington, D.C. Its intended recipient appears to have been the president.
According to Fox News, the FBI released the contents of the letters.
The bulletin said both letters included the phrase: "To see a wrong and not expose it, is to become a silent partner to its continuance."
Both were signed, "I am KC and I approve this message."
It's not clear why Sen. Roger Wicker, the junior senator from of Mississippi, was one of the targets. He was one of the few Republicans to support moving forward on a package of gun legislation, but that vote occurred later in the week than the letters were apparently sent.