The 27-year-old suspect was later shot and killed in a parking lot by responding officers.
"We believe this is a domestic violence homicide," Federal Way Police Chief Brian Wilson said at a news conference.
Police encountered a chaotic situation in Federal Way on Sunday night when they responded to reports of gun shots.
The suspect confronted arriving officers with a shotgun in a stairwell then fled to the parking lot after officers fired at him, Wilson said.
He was killed on the ground while reaching for a handgun, Wilson said.
"This is one of the most dangerous ... calls for law enforcement to respond to," Wilson said of the active shooter situation.
The names of the shooter and victims have not been released.
However, Wilson said the victims included the suspect's 25-year-old girlfriend, and three men, including the 62-year-old neighbor. The other men were 24 and 46.
Police were still piecing together information but believe the suspect shot his girlfriend in their apartment then killed the two younger men during an argument after he left the unit.
The relationship between the shooter and those two men wasn't clear.
Wilson said the 62-year-old neighbor heard the commotion from another apartment, went outside to investigate and urged others to call authorities. The man returned to his residence before he was shot and killed, the chief said.
Police said the gunman used the shotgun to fire on the man's door to gain entry.
Police said it did not appear that the older man knew the shooter.
Authorities received the first reports of suspicious circumstances about 9:35 p.m. Sunday.
"When officers arrived there were still shots being fired," said Federal Way police spokeswoman Cathy Schrock.
Police swarmed and searched the large apartment complex off I-5 before saying they were confident there were no more casualties.
By dawn, a King County medical examiner's office truck had arrived at the scene to pick up bodies while crime scene investigators kept working. One officer was seen carrying books and a gun to what appeared to be an evidence van.
A total of eight officers fired their weapons, Schrock said. All have been placed on administrative leave, which is standard policy in such situations, as the investigation continues.
Wilson said the suspect, who had a valid concealed weapons permit, had no criminal history. However, he had been involved in two prior domestic violence calls in Federal Way and Seattle, Wilson said.
Both incidents were verbal in nature and no arrests were made, Wilson said.
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