The report, dated October 29, 2001, ranked the threat of attacks against chemical plants second only to widespread use of biological weapons, such as contaminating the country's water or food supply, which could result in as many as 4.18 million casualties.

The study did not address the threat of a nuclear attack.

At least 123 U.S. plants stockpile amounts of toxic chemicals that could endanger more than one million people, if the chemicals were released and formed lethal vapor clouds, according to an EPA study last year.

The American Chemistry Council recently required members to complete security studies and correct vulnerabilities, using guidelines issued by the Justice Department.