Tukwila Police Department press release:

On 3-17-12 shortly before 3am, Tukwila Police officers responded to an apartment located in the 11600 block of 42nd Avenue South in Tukwila on a report of an unconscious, bleeding female.

Officers found the victim in the apartment, unconscious and suffering from stab wounds. The victim – a 17-year-old SeaTac girl – was transported to Harborview Medical Center by Medic One but was pronounced dead a short time later.
A 17-year-old male acquaintance who lived at the apartment was detained and questioned by Tukwila Police Department detectives and was eventually arrested and booked into the King County Youth Services Jail and charged with murder.

No motive for the crime has been uncovered.