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A couple in their 20s and their baby son were killed when a concrete slab from a sidewalk construction project fell from a bridge onto their truck in Washington state, police said Monday night.

Police in Bonney Lake, about 15 miles southeast of Tacoma, said a contractor was working on a section of the permanent bridge barrier on State Route 410 when the entire approximately 80- to 100-foot section detached and fell Monday morning.

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"It was immediately clear to all the first responders that arrived on scene that it was an unsurvivable event," Bonney Lake Police Officer Todd Green said.

For most of the day, police reported that one person had been killed, but as the rubble was cleared Monday night, three bodies were discovered.

The family wasn't immediately identified. Police said they lived in the area. The cause of the accident was under investigation by police and state labor safety authorities.

Mayor Neil Johnson's office said the project was intended to provide a safe walkway for pedestrians to cross the busy bridge. The contractor was identified as WHH Nisqually Federal Services, a construction business run by the Nisqually Indian Tribe, according to licensing records on file with the Washington secretary of state's office.

The company said it was investigating but would have no comment. State records showed it has had no citations or violations recorded since its founding, but the records don't compile any actions against affiliated subcontractors.

"I could hear three snaps, and down it went on top of the truck," a witness, Dawn Nelson, told NBC station KING of Seattle. "There was nothing anyone could do. It was just surreal."

A concrete slab from a sidewalk construction project fell from a bridge onto a truck, killing a couple and their baby son in Bonney Lake, Wash. on Monday. NBC News

SOCIAL

Young family dies after concrete barrier falls onto truck http://t.co/6g448vx1NI via @KING5Seattle — Becky May (@BeckyShockMay) April 14, 2015

Extrication of damaged vehicle completed. Sadly, 3 victims located. Victims include male, female in mid 20s, and infant male child. — BonneyLakePolice PIO (@BLPoliceDept) April 14, 2015