The US Centers for Disease Control and Prevention is investigating 109 childhood cases of a mysterious form of hepatitis, including five reported deaths, the agency has said.
"Investigators both here and abroad and around the globe are working hard to determine the cause," said Jay Butler, deputy director for infectious diseases for the CDC.
Fourteen of the children required liver transplants.
Dr Butler said around half of the 109 children diagnosed with hepatitis were also infected with a type of adenovirus, a virus that causes the common cold, but the agency is still investigating the exact cause of the illness.
The World Health Organization earlier this week said it had received reports of at least 228 probable cases from 20 countries, including a small number in Ireland, with over 50 additional cases under investigation.
The CDC said it is working with counterparts in Europe to understand the cause of the infections that can cause liver damage and lead to liver failure.
The CDC issued a health alert last week notifying doctors and public health authorities to be on the lookout for similar cases, and began examining case histories extending back to 1 October 2021.
"Because of the link to adenovirus, I would call that top of the list of viruses of interest," said Dr Butler.
"But we don't know if it is adenovirus itself that is causing the cases, or is there an immune reaction to this particular strain of adenovirus."
Hepatitis linked to this type of adenovirus has almost exclusively been associated with immunocompromised children, but many of the cases first reported to the CDC did not have immunocompromising conditions, Dr Butler said.
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Environmental factors are also being examined - such as the presence of animals in the house, as well as whether other pathogens, such as Covid, may have played a role.
Adenovirus cases might also be rebounding after Covid lockdowns stopped the spread for a few years, or the adenovirus might have evolved into a newer, more dangerous strain.
But the CDC has said it does not believe Covid vaccinations are to blame.
Nine cases in Alabama that were investigated in depth were among children with a median age of two years old, too young for a Covid vaccination.
