Factbox: Prior anthrax, ricin scares in the United States

(Reuters) - As many as 75 U.S. government scientists may have been exposed to live anthrax bacteria after failing to follow proper safety procedures, prompting an investigation by federal authorities.

Below are prior incidents involving such dangerous organisms:

* The most prominent anthrax attacks in the United States came by mail in 2001, shortly after the Sept. 11 airliner attacks on New York and Washington.

Letters containing anthrax spores were mailed to two Democratic Senators - Tom Daschle and Patrick Leahy - and to three TV networks and two newspapers. Five people died and 17 others were sickened by contact with the tainted mail. Two of the dead were postal workers infected while processing the mail.

The leading suspect, a U.S. scientist named Bruce Ivins, committed suicide in 2008, before he was charged with any crime.

* In 2003, an anthrax scare led the U.S. Postal Service to close 11 post offices in Washington DC, Maryland and Virginia after air quality monitors detected possible traces of anthrax spores at a Washington facility.

* The postal system has also been used to deliver ricin, another potentially deadly toxin. In 2004, an envelope containing ricin was discovered in Senator Bill Frist's mailroom in Washington, DC.

*In April 2013, an envelope addressed to President Barack Obama mailed from Memphis, Tennessee tested positive for ricin.

A second letter also from Memphis and intended for Mississippi Senator Roger Wicker was discovered at a mail processing facility before it could reach its intended target. It also tested positive for ricin.

(Reporting by Bill Berkrot; Editing by Richard Chang)