An FBI official confirmed the confidential alert had been dispatched across the country -- under the title "Improvised Chemical and Biological Agents" -- but declined to give any details.

Time magazine, in its online edition, said the alert was prompted in part by the discovery of homemade biological toxins in a radical Islamist compound in northeastern Iraq that was raided by US and Kurdish troops late last week.

According to Time, the bulletin warned that lethal agents like ricin could easily be manufactured using Internet recipes and equipment that can be bought from nurseries, hardware stores and mail order chemical supply houses.

"Recent events ... indicate an ongoing interest among terrorists in developing improvised weapons of mass destruction," the FBI alert said.

It also listed telltale signs of toxin production, such as large caches of yeast or infant formula, which can be used to grow or dilute biological toxins, and sacks of castor beans, from which ricin is extracted.

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