Chicago-based United Airlines is in hot water after a dress code incident Sunday morning.

In a series of tweets, activist Shannon Watts explains how two young girls wearing leggings were told to either change their attire or be denied boarding of flight 215 from Denver to Minneapolis.

Her first tweet, coming at 6:36 a.m. local time, said:

The tweet was preceded minutes later with:

She commented on the incident later saying “This behavior is sexist and sexualizes young girls. Not to mention that the families were mortified and inconvenienced.”

“As a mother of 4 daughters who live and travel in yoga pants, I’d like to know how many boys @United has penalized for the same reason. Their father, who was allowed to board with no issue, was wearing shorts.” she added.

After a massive backlash from Twitter users and travelers alike, United Airlines responded citing the airlines’ dress code that can be found in their “Contract of Carriage” that says “passengers can be refused to board if not properly clothed.”

The airline later announced the two passengers effected were “pass-riders” who are “United employees or their eligible dependents standing by on a space-available basis.” The airline explains that “pass-riders” travel by a separate set of rules that will not be released.

The incident has caught national attention, with the story trending on Twitter with over 100,000 tweets. The airline is investigating.