WASHINGTON, D.C. — Federal authorities are using DNA analysis to try to glean clues about the source of poisonous ricin powder posted to Senate Majority Leader Bill Frist’s office two weeks ago.

A federal law enforcement official last week told the Associated Press the letter contained a small vial of ricin and was addressed to the White House. It was intercepted Nov. 6 by the Secret Service and bore a postmark from Chattanooga, Tenn.

The letter was nearly identical to one found at a mail-sorting facility in Greenville, S.C., Oct. 15. The letters, signed “Fallen Angel,” complained about the new rules concerning the hours a truck driver can work.

Authorities are also testing to ricin to see if they can figure out where the castor plants used to make the poisons were grown, the Washington Post reported.