Washington ? New rules announced Monday will make it easier to investigate a bioterror attack on the U.S. food supply, though they won’t change the underlying problem: the vulnerability of the nation’s food.

The vulnerabilities were highlighted last week by Health and Human Services Secretary Tommy Thompson, who said he worried “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.

Thompson singled out the possibility that infected food would be imported from the Middle East, but experts say the threat is equally serious for food produced domestically.

“There are any number of threats, and they range from what’s done across the oceans to what’s done in the kitchen in the restaurant that you’re eating in,” said Michael Osterholm, associate director of the National Center for Food Protection and Defense at the University of Minnesota, a project of the Department of Homeland Security.

The regulations announced Monday by the Food and Drug Administration aim to trace the source of food contamination after the fact. Many businesses involved in the nation’s human and animal food supply — farms and restaurants are exempted from the rules — will have to keep records showing where they received food and where they shipped it.

Large companies have a year to comply with the new regulations, while smaller companies can wait up to two years.

The idea is to help investigators figure out where in a long chain a particular item may have been tainted.

“The ability to trace back will enable us to get to the source of contamination,” said Lester Crawford, acting FDA Commissioner.