West Precinct patrol officers first responded to a call about a shooting or stabbing just before 8 p.m. near Second Avenue and Pine Street.
When they arrived, officers found a teenager who had been stabbed five times in the back. He was taken to Harborivew Medical Center with life-threatening injuries.
Police say most of the people who saw the stabbing are suspected gang members who refused to cooperate with investigators.
However, one woman did approach a Tukwila Police officer at the Light Rail station near the airport to say she'd seen the stabbing. She pointed out two people who she said where involved in the attack.
Tukwila officers detained the two suspects until Seattle Gang Unit detectives arrived to interview them. Detectives later arrested one of the suspects, a 21-year-old, and booked him into King County Jail on suspicion of assault.
The investigation is ongoing.