Nine children in Alabama with severe acute hepatitis, three of whom suffered liver failure, all tested positive for adenovirus and none of them had a history Covid-19 infection, according to the Centers for Disease Control and Prevention.

The CDC on Friday published its most detailed findings so far about the children, after issuing a nationwide health alert last week. It said adenovirus infection may be an underrecognized contributor to liver injury in otherwise healthy children but further investigation is needed.

While hepatitis is not uncommon in children, the cluster of cases in Alabama surprised physicians because the previously healthy kids had severe symptoms and did not test positive for hepatitis viruses.

Public health authorities in the U.S. and Europe are closely tracking cases of severe hepatitis in kids after the U.K alerted the World Health Organization earlier this month about a cluster of cases there. The WHO has identified 169 cases worldwide so far, with the overwhelming majority of them in the U.K.

All nine children in the U.S. were patients at the hospital Children’s of Alabama, who ranged in age from about 2 to 6 years old, according to the CDC. Three of the patients suffered liver failure and two needed liver transplants. All of them have either recovered or are recovering.

The children’s symptoms before hospital admission included vomiting, diarrhea and upper respiratory symptoms. Eight of the patients had scleral icterus, a yellowing of the white of the eye. Seven had enlarged livers, six had jaundice and one had encephalopathy, a broad term for disease of the brain.

All of the children tested positive for adenovirus, a common infection that can cause respiratory illnesses, an upset stomach, pink eye and bladder inflammation or neurological disease in rarer cases. Adenovirus is a known cause of hepatitis in children with weak immune systems, but the patients in Alabama all had normal immune systems and no significant health conditions, according to the CDC.

Although six of the kids also tested positive for Epstein-Barr virus, the CDC does not believe these were acute infections because they tested negative for antibodies. The children all tested negative for hepatitis viruses A, B and C, according to the CDC. None of them had a history of Covid-19 infection.

Doctors in Alabama identified the first five cases last fall. The CDC and the Alabama Department of Public Health began an investigation in November. They identified four more cases in Alabama through February of this year. No additional cases have been identified in Alabama since February.

The CDC said it’s monitoring the situation closely to better understand the cause of severe hepatitis in the kids and to find ways to prevent the illness. The public health agency told physicians to be aware that whole blood tests, rather than plasma, might be better at detecting the presence of adenovirus.