A newly declassified letter reveals that top scientists in the Manhattan Project, the World War II effort to invent the atomic bomb, considered a plan to poison half a million of the enemy with radioactively contaminated food.

The plan, which appears never to have been carried out, is mentioned in a letter, dated May 25, 1943, from J. Robert Oppenheimer, who directed the Los Alamos bomb laboratory in New Mexico, to Enrico Fermi, a Manhattan Project researcher who, a year earlier, had achieved the first controlled nuclear chain reaction.

In the letter, Oppenheimer refers to the plan as if it were Fermi's idea, discusses various technical problems to be solved and concludes:

"I think we should not attempt a plan unless we can poison food sufficient to kill a half a million men, since there is no doubt that the actual number affected will, because of non-uniform distribution, be much smaller than this."

The letter does not give details on how the poisoning plan would be carried out but does suggest that strontium, a radioactive byproduct of atomic fission, "appears to offer the highest promise." Presumably the strontium would somehow be mixed into food supplies destined for the tables of Germans or Japanese.

The letter was found by Barton J. Bernstein, a Stanford University historian who specializes in science and technology policy.

Bernstein describes his find and its implications in the May/June issue of Technology Review magazine. Bernstein said he came across the letter while perusing recently declassified documents at the Library of Congress.

The letter, Bernstein wrote, "illustrates an important fact: Amid the horror of World War II, including German concentration camps and the mass killing of Jews, many U.S. scientists, like rank-and-file civilians, were willing to devise new ways to kill the enemy by the thousands and even hundreds of thousands."

The plan is not mentioned in any history of the Manhattan Project. Neither, Bernstein wrote, was the plan remembered by any of 16 leading Manhattan Project scientists still living.

"Most of these scientists," he wrote, "have forgotten or never knew that in 1941 a scientific advisory committee to the National Academy of Sciences suggested that the United States develop the radioactive products of fission as weapons."

The committee even gave the pursuit of such weapons a higher priority than the quest for the bomb itself.

Bernstein said it was not clear from the letter that Oppenheimer supported the plan.

By suggesting that the poisoned food be able to kill half a million people, Oppenheimer may have been trying to impose a nearly impossible goal in hopes of stopping the plan.

"Or," Bernstein wrote in his magazine article, "he may have lacked, or already overridden, personal doubts about the ethics of mass killings; he may have been troubled only by technical matters of efficacy and access to resources."

Oppenheimer died in 1967. His younger brother, Frank, also a physicist, called the letter "bloodthirsty."

"In those days," Frank Oppenheimer recalled, referring to World War II, "we talked about everything, any way of killing."