LONDON (AP) — European health officials investigating an outbreak of salmonella linked to chocolate Easter eggs that has sickened at least 150 children across the continent said Tuesday they suspect it is due to bad buttermilk in a Belgian factory.

In an assessment of the continuing outbreak, experts at the European Center for Disease Prevention and Control and the European Food Safety Authority said they had matched the same salmonella strain currently infecting people to samples taken from a factory in Belgium last December.

Officials said “the processing step involving buttermilk” was identified by the company as the point of contamination for two products, chocolate eggs that normally have a surprise toy inside and bite-sized praline chocolates. National authorities have previously named the involved company as the Italian chocolate firm Ferrero.

Before the Belgian factory was shut down, however, European officials noted it had exported “the implicated chocolate products” across Europe and globally.

As of this week, 150 cases of salmonella have been reported in nine European countries and the U.K. after the first case was identified in Britain last December. The majority of cases have been in children under age 10 and “an exceptionally high number” have been hospitalized, European officials said. Britain has so far reported the most cases – 65 – of whom 43% are hospitalized.

Ferrero began recalling chocolate eggs and other products in Britain, France, Germany, Italy and elsewhere weeks ago. Last week, the company expanded its recall to the U.S., acknowledging that some of the goods on sale were made in the tainted Belgian factory.

“This outbreak is rapidly evolving, and children have so far been most at risk for severe infection among reported cases,” the European CDC said in its report. It noted that further investigation was needed to determine how the problem happened and evaluating “the possibility of the wider use of contaminated raw material in other processing plants.”

Salmonella typically causes symptoms including diarrhea, fever and stomach cramps. Most people who get sick do not need any medicine but severe cases that result in hospitalization may require antibiotics or other treatment.

European officials warned that it was likely cases were being missed in some countries due to a lack of surveillance and genetic sequencing.

Ferrero has previously said it is cooperating with national and European health authorities in the outbreak investigation.

“We are taking this extremely seriously as consumer care is our top priority,” the company said.

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