The number of people suffering psychologically induced symptoms could far outweigh the number of actual victims in a chemical, biological or nuclear incident, according to a confidential Department of Homeland Security briefing document.

"Mass psychogenic illness" can "spread rapidly throughout a population," says the briefing, citing incidents in California in 2003 and Chechnya in 2005.

The briefing, prepared in 2006 but only leaked last week, defines mass psychogenic illness as "a phenomenon in which social trauma or anxiety combines with a suspicious event to produce psychosomatic symptoms, such as nausea, difficulty breathing, and paralysis."

In October 2003, the briefing says, bank customers and staff in California became ill, with symptoms including "headache, fever, faintness, and numbness in extremities" after a man sprayed an aerosol can into the air -- "although subsequent investigation determined that no chemical or biological agents were present."

"Observed symptoms might have been psychosomatic," the paper quotes investigators as concluding.

The document, posted last week by Wikileaks.org, highlights the fact that officials' public statements during chemical or biological events such as a suspected terrorist attack can easily make matters worse -- and that their ability to make them better is crucially reliant on a credibility with the public that many fear is eroding.

George Foresman, the Homeland Security undersecretary for preparedness until last year, questioned whether "the average citizen" would "trust what the government tells them about the likelihood that they've been exposed or not been exposed to a certain pathogen or a chemical."

"The trust between the American people and those who are in positions of authority and responsibility is not as strong as it needs to be," he told United Press International. "People are cynical" about government communications "especially about terrorism."

Foresman said public fears were likely to be greatest when confronted with "something where you cannot see easily whether you've been affected or not" -- for instance in the case of a release of a biological agent.

"The antidote to that fear is guidance and information," he said.

"Government (communication) has got to be direct, it's got to be quick and it's got to be exact," Foresman said, adding that "the officials (delivering it) need to be credible," which would often mean local public health, emergency management or law enforcement figures, rather than more senior elected officials like "a governor or a president."

He said this kind of crisis communication required "in-depth discussions with the local and national media about what you know and -- to be honest -- what you don't know."

Jim Harper, director of information policy studies for the libertarian Cato Institute, said the U.S. government had failed to do necessary strategic thinking about reassuring people in the event of an incident.

Psychosomatic effects aside, many experts believe the other effects of panic during a chemical or biological attack -- the impact on transportation systems and the nation's economy, for example -- could far outweigh the direct effects on the victims and their community.

There was "a lack of truly strategic planning," said Harper. "What kind of communications will reassure people that their society is not under threat" of destruction?

"It's got to be more than just 'run for the hills,' even if you're saying 'these are the specific hills you should run for,'" he concluded.

"You have to give people a sense of control," said Paul Slovic of Decision Research, a risk-perception specialist. "Either the sense that their government is in control, is handling it �� and/or explicit information (about the possible effects of any attack) which will enable them to take control themselves."

The leaked homeland security briefing recommends that the possibility of psych-somatic symptoms be taken into account in incident planning and response scenarios. Officials should ensure they are "communicating safety and security measures taken by the government and industry to defend against attacks; educating the public on the nature of biological and chemical attacks so they can accurately identify symptoms; (and) ensuring a quick response to real or psychosomatic outbreaks to isolate affected individuals and reassure the public."

The department declined to comment, but Foresman said he believed Homeland Security and the federal government as a whole had made progress on the communications and response issue, compared with the way the anthrax attacks had been handled in the fall of 2001.

He said the response to Hurricane Ike had been a model of what he called "joint information centers" -- where officials from different agencies could cooperate on developing a common operation picture and sharing it with the public.

Because of that, Texas Gov. Rick Perry "was able, in one briefing, to cover the entire range of issues they were dealing with," said Foresman, adding that a range of local officials had participated in the briefings alongside Perry to address detailed questions.

But "the biggest improvement of the last six or seven years," said Foresman, was on the task of "getting the technical data and turning it into plain, simple, understandable language."

The Department of Homeland Security "as a whole and government as a whole is doing much better," he concluded.

"Doing a better job with a smaller hurricane is not good enough," retorted Harper.