"When officers arrived there were still shots being fired," said Federal Way police spokeswoman Cathy Schrock.
Police saw two men lying wounded in a parking lot. They tried to rescue the injured men, but one reached for a gun, leading police to open fire, killing him.
The second wounded man also died, and police found a third body in the parking lot, as well as a man and a woman, both dead, in two separate apartments inside the complex.
Schrock said police were trying to determine if the woman was hit accidentally.
No officers were injured in the incident.
Eight were placed on administrative leave after firing their weapons, which is standard procedure for officers involved in shootings.
There was no immediate reason for what triggered the shooting, but Schrock said it was a gun battle among the four men who were killed.
"We're going to continue to go door to door in hopes that we can find some additional witnesses, and hopefully we won't be finding any more victims." Schrock said. "We still don't have any idea what started this disturbance tonight."
Police flooded the area and determined there are no additional shooters on the loose and that the public is safe.