(Graphic made with photos from energy.gov)

Federal authorities believe a food-borne attack on the U.S. homeland is unlikely, but they are still urging businesses and local law enforcement to keep watch for such an attack and a smorgasbord of other poison-based threats, including the possible contamination of skin products or even handrails at public places, according to documents obtained by Fox News.

This month — as part of its growing “If you See Something, Say Something” campaign — the Department of Homeland Security sent a document to restaurant and hotel owners in at least one major U.S. city, urging them to look out for, among other things, “persons conducting surveillance of … salad bars, condiment areas, and open bulk containers.”

“Improved awareness can prevent businesses from being used in illicit or terrorist activities,” the DHS document said.

Last month, DHS and the FBI jointly sent a threat assessment to local law enforcement and emergency responders across the country, warning that terrorists “continue to express broad interest in toxins and poisons that could be used to contaiminate food or water supplies, or spread through skin contact.”

“Widely circulated terrorist manuals contain instructions for mixing toxins and poisons with skin penetrating enhancers such as oils, lotions, and other ointments,” the threat assessment said. “Most toxins and poisons mentioned in terrorist manuals are more suitable for assassinations and small-scale attacks than for mass casualty attacks, but terrorists might calculate that a coordinated series of simultaneous attacks could produce comparable casualties and widespread public fear, as well as erode consumer confidence.”

Specifically, the threat assessment said, “perpetrators” could try to apply “foreign substances” to “items commonly touched by the general public,” including escalator rails and doorknobs, or they could try to access facilities where skin care products or foods are produced or stored — “possibly with help from insiders.”

The assessment mentioned ricin and cyanide as examples of toxins that could be used, and it suggested looking out for “suspicious behavior suggesting individuals may be adding unusual materials to buffet carts, salad bars or food preparations in open dining areas.”

As reported earlier this week, federal authorities obtained information “a while ago” from a “sensitive source” indicating associates of Al Qaeda in the Arabian Peninsula — or AQAP — were talking about targeting food at hotels and restaurants inside the United States, perhaps slipping harmful agents into salad bars or buffets.

While that “is something they are talking about,” it is “not something that we think is operationally in the mix,” one counterterrorism official said, adding that there is no such ”credible imminent threat” against the United States. Several sources said the counterterrorism community believes AQAP does not have the capability to successfully launch a food-borne attack.

But DHS has characterized such an attack as an “enduring threat.”

In August, DHS issued a “protection note” to federal, state and local law enforcement, assessing the risk of an “intentional contamination” of the U.S. food supply and offering guidance to protect against or respond to such an incident.

“While DHS has no information indicating an imminent threat to the U.S. food supply, we assess that terrorists’ past intentions and capabilities could indicate an enduring threat,” said a “key finding” of the DHS note. “The Food and Agriculture Sector continues to be an attractive target due to the potential for an attack to cause panic and to affect the Nation’s economy and public health.”

The note said that even “inexperienced individuals could produce or obtain material … suitable for a limited food contamination,” adding that “the overall vulnerability of the [food] Sector is increased due to the open and concentrated nature of modern U.S. farming and food processing practices, driven by large-scale production and distribution objectives.”

The food and agriculture industries account for one-fifth of the nation’s gross domestic product, according to the note, and an attack on those industries “could cause cascading consequences in the Healthcare and Public Health, Emergency Services, and Transportation Sectors producing economic and public health impacts of unknown severity.”

Just economically, a widespread food contamination incident could result in billions of dollars in direct and indirect losses, the note said.

The note recommended more than a dozen measures that local officials could take to help prevent or respond to such an incident, and in the event of such an attack, “affected areas of the system” or “suspect food stocks” may need to be isolated, according to the note.

Nevertheless, a food-borne attack in the U.S. homeland is not without precedent. The “single largest bioterrorism attack in the United States” occurred in 1984, when followers of a religious sect sprayed Salmonella bacteria onto salad bars at 10 restaurants in Oregon, poisoning 751 people, according to the note. The sect hoped to boost their own candidate in a local election by “incapacitating” most voters, the note said.

A few months ago, in the wake of the chatter related to AQAP, government representatives briefed a small group of officials from the restaurant and hotel industries on the threat information. Officials from DHS, the Department of Agriculture, and the Food and Drug Administration described the AQAP threat to industry insiders as “non-credible,” according to one private sector source with knowledge of the briefings.

Despite no credible threat, industry insiders welcome such briefings, according to one private sector source.

“When we get a flashing light, we pay attention, even if it’s not credible or non-specific,” the source said. “We make sure that we remain aware and vigilant.”

The source said DHS and other agencies are “doing a good job.”

“They keep us well-informed as much as possible as to what’s going on,” the source said. “They do a very good job trying to bridge the gap that was there pre-9/11 in terms of sharing information with the private sector.”

As for AQAP, White House chief counterterrorism adviser John Brennan recently described the Yemen-based group as “now the most operationally active node of the Al Qaeda network.”

“The group’s leadership clearly seeks to apply lessons learned from past attacks, including those of other groups,” Brennan said Friday during a forum on Yemen at the Carnegie Endowment for International Peace in Washington. “And their definition of success – stoking fear, even if their attacks fail – portends more such attacks.”

On Christmas Day last year, Umar F. Abdulmutallab of Nigeria tried to detonate his explosives-laden underwear over Detroit. He was allegedly trained and equipped by AQAP. Ten months later, in October, two explosives-laden packages were sent from Yemen to the United States, but the explosives were intercepted overseas after Saudi intelligence officials shared information about the plot.