A letter containing ricin sent last year to the White House threatened to turn Washington into a “ghost town” if new trucking safety regulations went into effect, according to a copy of the letter released Feb. 23 by the FBI.

The letter is one of two signed “Fallen Angel.” It had an Oct. 17 postmark from Chattanooga, TN, and was discovered by the Secret Service at a Washington offsite mail processing facility in early November.

The White House letter was typewritten on what appears to be yellow legal paper. Although it was addressed to the White House, the letter begins with “department of transportation” and then says:

“If you change the hours of service on January 4, 2004, I will turn D.C. into a ghost town. The powder on the letter is RICIN. Have a nice day. Fallen Angel.”

A similar ricin-laced letter was found Oct. 15 at a mail processing facility in Greenville, SC. In both cases, the author complained about new regulations that mandate more periods of rest for long-haul truckers.

The South Carolina letter also claimed that the author was the owner of a tanker fleet company and had access to large amounts of pulp from castor plants, which are the source of the poison ricin.

Investigators are also trying to determine the source of a small amount of ricin found earlier this month on a mail-opening machine in an office of Senate Majority Leader Bill Frist, R-TN. No envelope or threatening letter has been found in that case.

The FBI, Postal Inspection Service and Transportation Department are offering a $100,000 reward for information leading to the arrest and conviction of the author of the threatening letters. The FBI is also operating a toll-free tip line in the case at 1-866-839-6241.