A group of black teenagers was asked to leave an Apple Store in Melbourne, Australia after an employee responded to allegations that the teens might steal something. According to a report from CNET, a video of the interaction has gone viral and sparked a wave of backlash from people around the world.

Apple CEO Tim Cook responded to the situation by writing a letter to employees, saying that the incident was “unacceptable” and that the company was “open.” According to Cook’s letter, “One of our store employees gave an answer which shocked many of us.”

The video was posted on Tuesday by Melbourne resident Francis Ose, and it depicts the teens’ stunned response to being asked to leave the store for no apparent reason. The caption to Ose’s post read, “Simply Racism, made them apologise tho.”

This isn’t the first time Apple has come under fire for being racially insensitive. Denise Young Smith, an African-American Apple employee since 1997, was just added to the company’s executive leadership page in 2014. Smith is the vice president of worldwide human resources, and has vocalized the company’s ongoing struggle to become more diverse.

Cook maintains that the incident in Melbourne was isolated, but it underscores a larger trend in the way different people are treated by businesses around the world. The CEO said that the company is engaged in an ongoing effort to learn and grow, but critics maintain that they have a long way to go.

Cook’s letter to employees was obtained by Buzzfeed, and can be found here.