Health officials say they have detected more cases of a mysterious liver disease in children that was first identified in Britain, with new infections in Denmark, Ireland, the Netherlands and Spain.

Last week, British officials reported 74 cases of hepatitis, or liver inflammation, detected in children since January.

New infections have also popped up in the U.S.

Nine cases of acute hepatitis in children between 1 and 6 years old have been reported in Alabama.

The usual viruses that cause infectious hepatitis were not found in the cases, and scientists and doctors are considering other possible sources, including COVID-19, other viruses and environmental factors.

However, some experts in the U.S. are skeptical that COVID-19 is a factor. They say if it was, it’s likely there would be more hepatitis cases throughout the country.

British officials ruled out any links to COVID-19 vaccines, saying none of the affected children was vaccinated.

The U.N. has said it will continue to monitor cases. It predicts that more cases will be detected before they identify the cause of the outbreak.