MARTINEZ -- An assistant girls' water polo coach at Walnut Creek's Northgate High School has been charged with nine felonies alleging sex crimes against a teenage girl, according to the Contra Costa District Attorney's Office.

Christopher Yang, 27, of Walnut Creek, was charged last week with four counts of unlawful sex with a minor, two counts of sexual penetration of a minor, and three counts of oral copulation of a minor. The charges carry a punishment of up to eight years and four months in state prison.

Prosecutors are not releasing any details surrounding the allegations against Yang. The victim has asked for privacy, senior deputy district attorney Nancy Georgiou said.

Yang has also served as a swimming coach for Campolindo High School, and the Dana Hills Swim Team serving Concord and Clayton, according to the Dana Hills website. His online bio says that he started swimming at the Miramonte Swim Club at age 4 and then competed for the Diablo Valley College and the University of Hawaii teams.

Deborah Cooksey, an associate general counsel for the Mt. Diablo Unified School District, said Wednesday evening that Yang was a "seasonal coach" who was no longer employed by the district.

The district cooperated fully with the police investigation, Cooksey said. District officials are now conducting their own internal investigation, which they hope to have completed by the end of next week, she said.

Yang was arrested last week and is free from jail on bail. He is scheduled to be arraigned on the charges May 20.

Staff writer David DeBolt contributed to this report. Contact Malaika Fraley at 925-234-1684. Follow her at Twitter.com/malaikafraley.