WASHINGTON -- The government announced new rules Monday aimed at helping trace the source of food contamination, particularly in the event of a bioterror attack on the food supply.

Food manufacturers and others who work in the nation's human and animal food supply will have to keep records showing where they received food and where they shipped it next.

The idea behind the rules, announced by the Food and Drug Administration, is to help investigators figure out where in a long chain of supply a particular item of food may been tainted.

The regulations implement part of a law, passed after the 2001 anthrax attacks, which focused attention on many of the nation's vulnerabilities to bioterror attacks. Just Friday, outgoing Health and Human Services Secretary Tommy Thompson said he worries "every single night" about a possible terror attack on the food supply.

"For the life of me, I cannot understand why the terrorists have not attacked our food supply because it is so easy to do," Thompson said at a news conference announcing his resignation.

On Monday, however, Thompson was considerably more upbeat.

"Publication of this record-keeping rule represents a milestone in U.S. food safety and security," he said in a statement. "We have a lot of work yet to do, but our nation is now more prepared than ever before to protect the public against threats to the food supply."

The new rules affect anyone who manufacturers, processes, packs, transports, distributes, receives, holds or imports food. Officials at every step must keep records showing the chain of supply, including the immediate previous source of all food received and the next recipient of all food released.

"These records will be crucial for FDA to deal effectively with food-related emergencies, such as deliberate contamination of food by terrorists," said a statement from acting FDA Commissioner Dr. Lester Crawford. "The ability to traceback will enable us to get to the source of contamination."

Companies will be required to make these records available if the FDA has a reasonable belief that anarticle of food presents a serious threat.

Under the new rules, companies must retain records for periods ranging from six months to two years, depending on the shelf-life of the food. Records for animal food, including pet food, must be retained for one year.

Most companies have a year to comply with the new regulations. Small companies have longer.

The regulation is the fourth in a series of FDA rules implementing the 2002 bioterrorism law. Previous regulations required food facilities to register with the FDA and required those exporting food to the United States give American inspectors advance notice before shipments arrive.