Justin Bieber apologizes for racist joke

Story highlights

TMZ posts video of 15-year-old Justin Bieber telling a racist joke

Now 20, he says he's sorry for the "childish and inexcusable mistake"

Bieber: "I was a kid then and I am a man now who knows my responsibility to the world"

CNN  — 

Justin Bieber was in serious damage control mode Sunday after a video of him telling a racist joke to friends surfaced.

In the video, posted to TMZ.com, the pop star, then 15, asks: “Why are black people afraid of chain saws?”

He answers his own question: “Run n*****, n*****, n*****, n*****,” he says, imitating the sound of a chain saw. Then he laughs.

TMZ reported it got the video four years ago but decided against posting it then becaus