Eleven people have been hospitalised with Salmonella I 4,,12:b:- infection but no deaths have been reported.
The Centers for Disease Control and Prevention (CDC) said eight of 11 ill people interviewed had kratom in pills, powder or tea but no common brands or suppliers have yet been identified.
Kratom in many forms and not always labeled
The Food and Drug Administration (FDA) said products are marketed in many forms, including as leaves, pills, capsules, powder and tea and may not mention kratom on the label.
There are no FDA-approved uses for kratom and the agency has received reports about safety including more than 30 deaths associated with its use.
FDA and state and local health officials are attempting to identify specific brand names or suppliers to learn more about the source and route of Salmonella contamination.
Kratom is also known as mitragyna speciosa, mitragynine extract, biak-biak, cratom, gratom, ithang, kakuam, katawn, kedemba, ketum, krathom, krton, mambog, madat, Maeng da leaf, nauclea, Nauclea speciosa or thang.
The plant is consumed for its stimulant effects and as an opioid substitute and grows naturally in Thailand, Malaysia, Indonesia and Papua New Guinea.
Illness dates range from 13 October, 2017 to 30 January, 2018. Ill people range from six to 67 years, with a median of 41 and 16 are male.
The American Kratom Association (AKA) questioned the link - especially of a six year old taking such a product.
Illnesses after 23 January might not yet be reported due to the time it takes between when a person becomes ill and when illness is reported - an average of two to four weeks.
Most people infected with Salmonella develop symptoms 12 to 72 hours after infection and illness usually lasts four to seven days.
Whole genome sequencing on isolates from ill people are closely relatedly genetically, meaning they are likely to share a common source of infection.
The FDA and the Federal Trade Commission announced a crackdown last month on companies marketing dietary supplements to consumers trying to overcome opioid addictions.
However, kratom, did not form part of this enforcement action.