Concrete from a construction project crashed onto the family's car, killing Josh and Vanessa Ellis and their 8-month-old son, Hudson, authorities said.
"The damage was so severe it was impossible to tell how many victims were in the vehicle. The only thing we had was we could tell there was one victim," said Officer Todd Green of Bonney Lake Police Department, according to CNN affiliate KOMO.
It took nine hours Monday for crews to remove enough debris to discover the death toll on Angeline Road underneath State Route 410, authorities said.
Authorities continued their investigation Tuesday, but initial findings were that there was an ongoing construction project on the highway and a side jersey barrier "came loose and fell onto the roadway," Green told the station.
"When it fell off the overpass, it landed square on the roof of the vehicle," Green added.
Josh and Vanessa Ellis were youth pastors at the EastPointe Foursquare Church in Puyallup, near Bonney Lake, the church said.
The couple earlier wrote a short autobiography on the church website:
"We love to laugh. We are passionate about seeing young people discover the love and grace that Jesus abundantly pours out on them.
"We get really excited about good coffee, quality time with friends and Seattle welcoming an NBA basketball team back to our city," the couple wrote. "We love drive-in movies, frozen yogurt, dates to IKEA and trips to the beach."
Lead Pastor James Ludlow said his church was reeling.
"We are stunned! Shocked! Wounded, broken and dismayed. But we know one thing for sure ... they are in glory in the loving arms of our King Jesus!" Ludlow said on the church's Facebook page.
Motorist Dawn Nelson was driving behind the Ellis' vehicle.
"I was just a second or two behind him," Nelson told KOMO. "I could hear the three crunch sounds and then it just came down."
Neighbors recounted a violent collapse.
"I thought a semi had come down the guardrail here in front," neighbor Katie Vance told the station.
She felt the crash inside her home.
"It was a metal sound and a very heavy sound ... it was through your feet - like a guttural. It shook the whole house," she told the affiliate.