WASHINGTON — The Food and Drug Administration is building a legal case against companies that sell pure powdered caffeine, which can be fatal even in small doses.

The FDA warned consumers to avoid pure powdered caffeine this summer after the death of an Ohio teen. Some major retailers have stopped selling it in bulk, but the substance is still widely available on the Internet and in some stores.

Even a teaspoon of the powder can be lethal — it is equivalent to 25 cups of coffee. Some teenagers and young adults have been particularly drawn to the powder for its perceived energy boost.

Eighteen-year-old Logan Stiner of LaGrange, Ohio, died in May after consuming it. It was also linked to the death of a Georgia man this year.

Caffeine powder is usually marketed as a dietary supplement and is unregulated, unlike caffeine added to soda. Since the FDA doesn’t have the legal authority to pull such a substance off the shelves, the agency is building a legal case against those who market it in bulk in an attempt to persuade them to stop.

Michael Taylor, FDA’s deputy commissioner of foods, said in an interview with The Associated Press Monday that it is “inherently irresponsible” to market such a potentially dangerous product.