HALF MOON BAY -- A Richmond runaway girl whose body washed onto a Peninsula beach last month killed herself, the San Mateo County coroner said Tuesday.

However, because 16-year-old Cathy Qiu's corpse was partially decomposed, investigators could not determine how she took her life, San Mateo County Coroner Robert Foucrault said.

Cathy was in the water for up to five days before she was spotted Oct. 21 in the surf off San Gregorio State Beach, which is about 11 miles south of Half Moon Bay.

The honor student at Albany High School was reported missing Sept. 27.

Sheriff's investigators concluded that she killed herself, Foucrault said. No information was immediately available on the basis for their conclusion.

Contact Joshua Melvin at 650-348-4335. Follow him at Twitter.com/melvinreport.