The boy's dad was pulling him in a wagon on a trail along Highway 410 when a truck left the roadway near Jefferson Avenue and ended up on the trail, according to Buckley Police Chief Jim Arsanto.
The truck hit the boy and then continued down the trail until it hit a telephone pole.
The boy was rushed to Mary Bridge Hospital where he was later pronounced dead. The 78-year-old driver was taken to St. Elizabeth Hospital in Enumclaw to be treated for minor injuries.
"(It's a) tragedy any time we lose a life, let alone a small child," Arsanto said. "It touches home. My officers are having trouble working on this case"
Former firefighter and EMT Todd Kibbey was the first person on the scene after the crash.
"I heard bang, bang, and a loud one when he hit the pole. I jumped up and I ran outside, and I just heard 'baby' and I ran across the street," Kibbey said.
Investigators say neither drugs nor alcohol were a factor in the crash, but Arsanto said it appears the driver was moving at a "high rate of speed" and did not hit the breaks.
"He was unable to say how it happened, which led us to believe that something else was playing a part of this accident," Arsanto said.