Two youth pastors from Washington State died, together with their eight-month-old son, when a slab of concrete fell on their pickup truck on Monday.

Josh and Vannessa Ellis, who were in their mid-20s, and their son Hudson were killed by falling construction debris as they drove under a highway overpass at about 10.30 am.

Construction workers were building a pavement on the overpass, about 30 miles southeast of Seattle, when a concrete barrier fell onto the road below.

The family are thought to have died instantly, although it took nine hours to clear the site and determine the number of victims, CNN reports.

Dawn Nelson, who was driving in a car behind their truck, told KGW Portland: "We were just heading down the street...and I could hear three snaps and down it went on top of the truck.

"There was nothing anyone could do. It was just surreal."

Local police and the state transport department are investigating what caused the barrier to fall.

James Ludlow, senior pastor of EastPointe Foursquare Church, Bonney Lake, where the young couple worked, told Associated Press: "It's a tragic event. In the blink of an eye, inhale and exhale, and they're in the presence of God."

In a statement on the church website Ludlow added: "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! We WILL rise from these ashes and He will be glorified through this tragedy."

The church is holding a vigil to remember them this evening. A Gofundme account has been set up to raise funds for a memorial.

A tribute on the fundraising site reads: "Where you hear boisterous laughter, words of encouragement and see friendship is where Josh and Vanessa where always found. When Hudson came into their lives just 8 months ago, he fit right in with the most joyous personality ever!

"Josh and Vanessa youth pastored and lead worship to students and adults and it was their passion to see young people discover the love and grace that Jesus abundantly pours out on them. They never passed up an opportunity to share about the love of Jesus. We are all left with a huge whole and will miss the way they light up every room they walked into. We don't doubt that heaven just got a whole lot louder and they are being welcomed by all of heaven."