Officers responded to a report of gunshots at about 1:30 p.m. and could still hear shots being fired from the house when they arrived, police spokesman Sean Whitcomb said. A man in his 50s ran into the house. Two more shots were fired and he ran back out, telling officers his wife had shot herself.
When police were able to go inside, they found the four dead, including a woman in her 50s they believe was the shooter, Whitcomb said.
He said police don't believe anyone else was involved.
In addition to the older woman, the dead included two women in their late teens and a man in his 30s, police said. Their relationships were not immediately known.
Assistant Police Chief Jim Pugel said a wounded woman - who fled the house and is expected to survive - was able to speak to officers when they arrived.
"All she said was, `My mom has gone crazy,' " Pugel said.
The woman, who is in her 30s, was in serious but stable condition at Harborview Medical Center, hospital spokeswoman Susan Gregg-Hanson said Thursday afternoon.
Whitcomb said police would not release the names or ages of the deceased Thursday, adding that the information will come from the King County Medical Examiner.
Police said two handguns were recovered from the house in a lower middle-class neighborhood a block north of southwest Seattle's city limits. No motive for the shootings was known, officers said.
Neighbor Bobby Miller told KOMO that police showed up almost immediately after the shots were fired.
"I just heard gunshots. I come out and cops were all over," Miller said. "Just 'bam, bam' and that was it. Then screaming over here."
Travis Rowland, 53, said he was about a block away waiting to get his truck repaired at an auto shop when he saw a woman run from the house hollering.
As police arrived, Rowland said, a man "busted away from the police officers and ran into the house." Rowland said the man was inside for about 10 to 15 seconds before "there were a couple more shots, pow, pow, pow. At first I didn't know who was shooting. He came back outside."
Associated Press writers George Tibbits, Phuong Le and Donna Gordon Blankinship contributed to this report.