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Federal health investigators say they have more firmly linked an outbreak of salmonella food poisoning to kratom supplements.

At least 40 people in 27 states have been reported sick in the outbreak, the Centers for Disease Control and Prevention said Friday.

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No one has died, but 45 percent of those made ill have been hospitalized, the CDC said.

And the CDC said it found the missing link — the culprit bacteria in samples of kratom.

“The outbreak strain of Salmonella was found in leftover kratom powder collected from sick people in North Dakota and Utah,” the CDC said.

“Investigation findings link the outbreak to kratom products, but the evidence collected to date has not identified a common brand or supplier of kratom.”

The sale and use of kratom is controversial in the U.S. The Food and Drug Administration has issued warnings about the herb, saying it acts like an opioid drug and advising people to stay away from it.

The Drug Enforcement Administration is looking at stronger restrictions on the sales of kratom, which is currently traded freely on the internet and in some stores. Advocates say it works as a stimulant in small doses, but can help people overcome opiate dependency in larger doses. It’s sold under a variety of names, including Thang, Kakuam, Thom, Ketom, and Biak.

Salmonella is a very common bacteria that causes food poisoning. The CDC says it is not clear how it could have gotten into supplies of kratom. It's been linked to supplements before, and caused an outbreak in food powder in 2016.