UPDATE:
LEXINGTON, ky (AP) -- The Lexington Herald-Leader quotes U.S. Marshal Mike Klein that between 25 and 50 of the letters were mailed by an inmate at the Big Sandy Feral Prison in Martin County.

Tests showed that the substance in the letters was a powdered artificial sweetener.

ORIGINAL STORY
FRANKFORT, Ky. (AP) - At least four congressional offices in
Kentucky have received threatening letters that purported to
contain anthrax.

The offices of U.S. Reps. Geoff Davis, Brett Guthrie, Ed
Whitfield and John Yarmuth received the letters in the mail Monday morning.

The federal courthouse in Pikeville was shut down after the discovery of a suspicious substance Monday afternoon. Emergency workers prepared decontamination units outside the courthouse. The area has since been declared safe.

Guthrie spokesman Nate Hodson said a staffer in the Bowling
Green office opened one of the letters and immediately called
authorities. Hodson said the first line of that letter said the
envelope contained anthrax.

Davis spokeswoman Alexandra Haynes said a similar letter arrived
at the Fort Mitchell office. She said the letter apparently was
sent by a prison inmate. She didn't know which prison.

Hodson and Haynes said investigators determined that the anthrax
threat was a hoax.