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PHILADELPHIA (CBS) — Philadelphia police are searching for a 17-year-old boy accused of sexually assaulting a teen girl in the Somerton section of the city.

Police say the sexual assault happened on the 13000 block of Cardella Place on Sept. 6 around noon while she was on her way home from school.

According to police, the suspect approached the 13-year-old victim from behind, placed his hand over her mouth, and told her, “If you scream, I’ll kill you.”

“At approximately 12 p.m., the female victim was on her way home from school. I believe it was an early dismissal day, and she takes public transportation,” said Philadelphia Police Capt. Sekou Kinebrew. “She got off the bus in the area of Bustleton and Byberry and was walking home.”

Police say the suspect then dragged the girl into a gated yard near Cardella Place and Blakeslee Drive, and sexually assaulted her.

The suspect is described as a 17-year-old black male, 6 feet tall, weighing between 155 to 165 pounds, with a slim build, crooked nose and short black hair. He was last seen wearing a black t-shirt with orange writing on the front, and gray sweatpants.

Police say call 911 immediately if you see the suspect.