The chairmen of five key Senate committees on Thursday warned the chief executives of major convenience stores and wholesalers to stop sales of illicit flavored vaping products that they called “widespread violations of federal law.”

The senators voiced their concerns in letters to the companies, amplifying the frustration among some lawmakers in Congress over the continued availability of e-cigarettes in vivid colors and candy flavors that attract young people who could become addicted to nicotine. The unchecked sales, they wrote, “pose a tremendous public health threat.”