Texas’ health commissioner told lawmakers Monday they are still trying to determine the origin of a South Plains-Panhandle measles outbreak more than a month after the first patients reported symptoms.

The first measles death in the growing outbreak in West Texas was a school-aged child. The child was unvaccinated and had been hospitalized in Lubbock, Texas, said Lauren Adams, Lubbock city spokesperson. This is the first US measles death since 2015, when a woman in Washington state died.