It's a culinary mystery worthy of Agatha Christie.

Britain's Health Protection Agency said Friday about 400 people reported falling ill after eating at The Fat Duck, a world-renowned British establishment reputed for dishes such as snail porridge and bacon-and-egg ice cream. Health officials are baffled by the outbreak of diarrhea and vomiting while the celebrity chef, Heston Blumenthal, has said his restaurant will remain closed until the mystery is solved.

"It's so weird — we haven't found anything. But I can't take the risk of keeping open until we have exhausted every single avenue," Blumenthal told The Times of London last week, shortly after the first group of people reported falling ill.

Britain's Health Protection Agency is screening The Fat Duck's recent diners, and food samples are being tested at the agency's Center for Infections in London. So far, there are no answers.

"This is a very complex outbreak," agency official Dr. Graham Bicker said in a statement. An agency spokeswoman declined to elaborate except to say they had yet to receive any results.

Peter Harden, the coeditor of Harden's London and U.K. restaurant guide, said that while top restaurants were occasionally embarrassed by unclean kitchens or unsanitary staff, he had never heard of a restaurant closing under such circumstances.

"It's pretty much unheard of," said Harden.

Blumenthal, whose three-Michelin-star restaurant is arguably the most famous in the country, is known for using gels, enzymes, liquid nitrogen, and nontraditional techniques such as dehydration and vacuum cooking. Described on the restaurant's Web site as a culinary alchemist, the bald, bespectacled Blumenthal has regularly appeared on television, whipping up things like raspberry sorbet by mixing pureed raspberries with dry ice.

Harden said that while Blumenthal had a bit of a "mad scientist" image, his concoctions could wow even the most jaded palate.

Blumenthal's latest televised gastronomic adventure, "Heston's Feasts," includes stabs at absinthe jelly, deep-fried mealworms (injected with tomato-flavored paste for added gooeyness) and a pink "Alice in Wonderland"-inspired potion designed to taste like toffee, toast, custard, cherry pie and turkey.

By comparison, the food offered on The Fat Duck's $172 tasting menu is relatively conservative: Guests can choose from Nitro-green tea, parsnip cereal or roast foie gras "benzaldehyde."

The Fat Duck has been in business since 1995, and tables have to be reserved months in advance.

But something at the Duck went wrong in late January through mid-February.

Among the guests to fall ill was boxing promoter Frank Warren, who told Sky News television that "everything was fabulous about the evening."

"The food, the setting, the service, it was unbelievably good but unfortunately, afterwards, all of us were ill," Warren said. At first, the Health Protection Agency said 40 customers had fallen ill, but the agency said media attention prompted even more people to report getting sick.

With the restaurant closed since Feb. 24, the British media have spun out a host of theories to explain the sicknesses — an ill employee, bad stock or maybe — as The Daily Mail suggested — sabotage.

The Health Protection Agency said it ruled the latter out but did not explain why.

"It's straight out of Agatha Christie, isn't it?" Harden said.

Attempts to reach Blumenthal by e-mail through his restaurant and Channel 4 — which is broadcasting "Heston's Feasts" — were unsuccessful. No one picked up the phone at The Fat Duck on Friday. There didn't seem to be any activity inside the restaurant, a two-story cream-painted brick building in Bray, an affluent village about 50 miles west of central London.

Blumenthal retains a devoted following, according to Katrina Kutchinsky, who does public relations work for restaurants at Neil Reading Public Relations. She said he's unlikely to suffer professionally from what was generally perceived as a stroke of bad luck.

"I don't think it's called his amazing reputation into question," she said.

In any case, Blumenthal's show goes on. The next episode, featuring trained pigeons that burst out of a gigantic pie, airs March 10.