Walmart says it will stop selling electronic cigarettes at its namesake stores and Sam’s Clubs following a string illnesses and deaths related to vaping.

The Centers for Disease Control and Prevention said 530 confirmed and probable cases have been reported in 38 states and one U.S. territory, up from 380 a week ago but the cause remains unknown.

Meanwhile, the Food and Drug Administration revealed that its criminal investigations unit started tracking leads early on, as cases pointed to black market vaping products.