A badly mixed measles vaccine killed between at least 15 to 50 children on Tuesday in northern Syria and caused officials to halt the vaccination campaign while multiple groups investigate the cause.

The campaign was conducted by the Measles Control Task Force, a branch of an interim government that has taken control of parts of northern Syria during that nation’s three-year civil war. It’s meant to combat measles, mumps, polio and rubella in Syria. The group does not know whether the deaths were the result of an accident or deliberate action, according to a statement on its Facebook page, but it is looking into the matter. The World Health Organization has also opened an independent investigation.

The Measles Control Task Force said it had already provided the measles vaccination to some 20,000 children at the start of this week before officials began to hear reports of problems early on Sept. 16., according to the group’s statement. Though it reports that only 15 children have died, the group said it knows of 50 to 75 cases in which a vaccine had adverse effects on a child, and The New York Times reports that the bad vaccine has killed as many as 50 children.

The initial findings from the task force’s investigation show that the measles vaccine was mixed with a muscle relaxant that can be fatal when administered to small children, which would explain why children between 6- to 18-months-old have died while older children have survived. The children’s symptoms ranged from an inability to breathe to a slowed heart rate that can be fatal in infants.

The Measles Control Task Force has suspended its campaign indefinitely while it continues to look into the cause of death and how the vaccine came to be mixed with the muscle relaxer.

Juliette Touma, a communications officer for UNICEF, could not confirm any of the details provided in the task force’s statement, but expressed concern that the incident would delay an even broader vaccination campaign aimed at 25 million children across Syria and the Middle East.

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