Health officials announced Friday that sprouts from a farm in northern Germany caused the outbreak that has killed 31 people, sickened nearly 3,100 and prompted much of Europe to shun vegetables.
"It was like a crime thriller where you have to find the bad guy," said Helmut Tschiersky-Schoeneburg, head of Germany's consumer protection agency.
Health officials said they tracked the bacteria's path from hospital patients struggling with diarrhea and kidney failure, to the restaurants where they had dined, to specific meals and ingredients they ate, and finally back to a single farm.
There are more questions to answer, including what contaminated the sprouts in the first place: Was it tainted seeds or water, or nearby animals? The answer is still elusive.
Still, it was little surprise that sprouts were the culprit. They have been blamed in past food poisoning cases, including in Michigan and Virginia in 2005 and a large outbreak in Japan in 1996 that killed 11 people and sickened more than 9,000.
While sprouts are full of protein and vitamins, their growing conditions and the fact that they are mostly eaten raw make them ideal transmitters of disease. They require 100-degree heat and humidity -- precisely the same conditions E. coli needs to thrive. Sprouts have abundant surface area for bacteria to cling to -- and washing won't help if the seeds themselves are contaminated.
"E. coli can stick tightly to the surface of seeds used to grow sprouts and they can lay dormant on the seeds for months," said Stephen Smith, a microbiologist at Trinity College in Dublin. Once water is added to make them grow, the bacteria can reproduce up to 100,000 times.
Interviews with thousands of patients -- mostly women ages 20 to 50 with healthy lifestyles -- led investigators to conclude initially that salads could be the problem.
Health officials immediately warned consumers to avoid cucumbers, tomatoes and lettuce -- causing huge losses to European farmers as demand plummeted for their produce. But the seemingly ubiquitous alfalfa, radish and other sprouts weren't yet on anyone's radar.
"You get this stuff in every cafeteria," said Gert Hahne, spokesman for the Agriculture Ministry in Lower Saxony, the state where the contaminated sprouts were found.
"But after two weeks of diarrhea, most people don't remember if they had a few sprouts on top of a ham sandwich or mixed into a salad."
Inspectors visited more than 400 farms in Lower Saxony alone and the state put 1,000 people on the case, including health authorities, food inspectors and veterinarians.
Experts conducted microbiologic tests -- a total of 4,645 nationwide. They visited farms and checked their hygienic conditions, especially whether manure was used and could have contaminated produce.
Then on May 26, some pieces began to fall in place: Patients mentioned they had eaten sprouts and inspectors visited a small organic farm near the village of Bienenbuettel that grows many different types, including alfalfa, radish, onion, broccoli, garlic, linseed, wheat and sunflower varieties.
Although tests turned up negative -- a common result in E. coli investigations, when the offending food is usually consumed before the probe begins -- authorities started looking into the farm's delivery records.
That took them to a golf club in Lueneburg, a restaurant in Luebeck, another in Rothenburg/Wuemme and cafeterias in Frankfurt, Darmstadt and Bochum -- all places where customers had fallen ill.
The Robert Koch Institute, Germany's disease control center, questioned 112 people who had eaten at a single restaurant, including 19 who had fallen ill. All of the sick people had consumed produce from the suspect farm.
"They even studied the menus, the ingredients, looked at bills and took pictures of the different meals, which they then showed to those who had fallen ill," said Andreas Hensel, head of Germany's risk assessment agency.
The result was that customers who ate sprouts were nearly nine times more likely to be infected than other diners. Twenty-six clusters of sickened people were identified -- and another 30 are under investigation -- all connected to the farm.
Then came the nearly-smoking gun: On Wednesday, it was confirmed that three farm workers had fallen ill from E. coli in early May, when the outbreak first started.
On Thursday night, German medical and agriculture officials held a conference call.
"That's when we were told: 'Your sprout lead is foolproof,"' Hahne said.
Reinhard Burger, the president of the Robert Koch Institute, said the investigation produced enough evidence to pinpoint the sprouts as the source even though no laboratory tests came back positive.
"It was possible to narrow down epidemiologically the cause of the outbreak of the illness to the consumption of sprouts," Burger said Friday at a news conference. "It is the sprouts."
Burger warned the crisis was not yet over and people should not eat raw sprouts. While the Bienenbuettel farm was shut down last week and all of its produce recalled, some tainted sprouts could still be in the food chain.
Investigators were still testing seeds and other samples from the farm.
Officials in North Rhine-Westphalia state also reported Friday that tests had confirmed the deadly E. coli strain in a bag of sprouts from the farm that was in the garbage of a family near Cologne where two people had been sickened.
The outbreak has sickened nearly 3,000 people in Germany, with 759 of them suffering from a serious complication that can cause kidney failure. Twelve other European countries have 97 cases and the United States has three.
On Friday, authorities lifted the warning against eating cucumbers, tomatoes and lettuce, and Russia agreed to remove its ban on European vegetable imports. European farmers, forced to dump tons of unwanted produce, breathed a sigh of relief.
But consumers were not yet fully convinced.
"It is a relief to finally get some definite information," said Heinz Schirnig, a 74-year-old resident of Uelzen, near the contaminated farm. "But I don't know if we can trust this."
Angelika Peilert, a 59-year-old Berlin resident, agreed.
"I will not eat any fruit or vegetables until they have an ultimate proof," she said. "Only fruit like bananas which you can peel. The risk is still too big. I have a small grandson and I want to see him grow up."