The Federal Trade Commission (FTC) has announced that it has sent out 90 letters to celebrities, athletes, and other social media stars warning them that they need to clearly show when an Instagram post was sponsored by a brand.

This is the first time the FTC has reached out directly to social media stars but it didn’t disclose the list of people it contacted.

In the statement the FTC said: “In addition to providing background information on when and how marketers and influencers should disclose a material connection in an advertisement, the letters each addressed one point specific to Instagram posts – consumers viewing Instagram posts on mobile devices typically see only the first three lines of a longer post unless they click ‘more,’ which many may not do.”

The letters pointed out that a number of posts used hashtags, such as #sp or #partner, that may not be understood by consumers to designate an ad. The regulatory body has a set of rules, which define how sponsored content should be highlighted.

This isn’t the first time the FTC has put out a warning about sponsored content that hasn’t been properly disclosed on on Instagram. In March 2016 it filed a complaint against clothing brand Lord & Taylor, which sent out dresses to 50 fashion influencers and paid them to take photos wearing them. It has the power to impose financial punishments as it did with Warner Bros. in July 2016, when it found the media company paid social media influencers, including PewDiePie, but didn’t force them to disclose that they had been paid.

A search on Instagram for the #sp hashtag yielded over 10 million results, with many posts having the hashtag at the end of the description.

Celebrities, like Kendall Jenner, came under fire in August 2016 after the non-profit organization Truth in Advertising found over 100 posts that had no mentions of them being paid for.