WASHINGTON (AP) — Federal health officials on Tuesday announced a nationwide crackdown on underage use of a popular e-cigarette brand following months of complaints from parents, politicians and school administrators.

The Food and Drug Administration issued warnings to 40 retail and online stores as part of a monthlong operation against illegal sales of Juul to children. Investigators targeted 7-Eleven locations, Shell gas stations and Cumberland Farms convenience stores as well as vaping shops.

FDA regulators also asked manufacturer Juul Labs to turn over documents about the design, marketing...