"This is obviously a very tragic situation," said Seattle Police Deputy Chief Nick Metz. "It's not very normal that we have this level of folks who are victimized in this way."
The accident happened shortly after 4 p.m. in a residential neighborhood in north Seattle.
Seattle police spokesman Jeff Kappel said the driver of the vehicle, 50-year-old Mark W. Mullan of Seattle, was placed under arrest and processed for suspicion of driving under the influence of alcohol and/or drugs.
Court records show Mullan has had multiple DUI arrests in just the past several month. He was arrested for DUI on December 27, 2012, and again on January 14.
A 66-year-old man and a 68-year-old woman were pronounced dead at the scene, Kappel said.
Kyle Moore, Seattle Fire Department spokesman, said the injured were a 33-year-old female and a 2-week-old infant. The gender of the infant was not immediately known.
"When we arrived on scene, that infant was not breathing, so we initiated CPR and got the heartbeat back," Moore said.
The adult female suffered a serious head injury. Moore said both the woman and infant were in critical condition with "life-threatening injuries." They were taken to Harborview Medical Center. It was not known whether the woman was the infant's mother.
Kappel said detectives from the police department's Traffic Collision Investigation Squad were assisting with the inquiry.
"It's quite a workup that they do, they'll be out here for a couple hours," Kappel said.
Lacia Bailey lives in the area and said she spoke with the victims just seconds before the crash.
"I just talked to them," she said. "They had just moved here from Indiana when she had the baby."
As the family walked away, Bailey continued speaking with some neighborhood kids. Seconds later, everything changed.
"I looked up and black truck hit them and people went flying," she said. "All four of them were laying separately on the ground. Blood coming out of their heads."
The accident occurred across the street from a middle school, and Metz said police would be more visible in the area in the immediate future.
Moore said this accident was particularly jarring for first responders.
"It hits both police and fire ... they have kids, it hits them hard."