The chaos started just after 5 p.m. when a vehicle was driving the wrong way near 7200 West on the busy freeway and slammed into a semitrailer truck.
Investigators said the driver of the vehicle is believed to have been intoxicated, Utah Highway Patrol Sgt. Wade Breur said.
Three people were transported to different Salt Lake area hospitals in serious condition, including the driver.
About 45 minutes later, a semi barreled into eight cars just down the road, killing three people in three separate cars.
Officials did not release names of the deceased pending notification of family. As of 9:20 p.m., one woman was still trapped in her vehicle awaiting extrication. Four others were transported to the hospital, including one in extremely critical condition.
"Everyone was tied up on the first one, and then this happens," Breur said.
Investigators don't know why the truck driver slammed into the back of the vehicles, Breur said. Traffic was considerably slower after the first crash occurred.
Witnesses said it was like watching a bowling ball hitting pins.
Three medical helicopters were called to the scene, shutting down westbound traffic for about an hour.
The crash shut I-80 eastbound down for hours, as the semi tipped and sprawled out across the eastbound lanes. As of 10 p.m., those lanes were still closed.
The traffic accidents didn't end there. In between the two major crashes, three minor accidents occurred on the same stretch of road.
Then after the semi slammed into the eight cars, crews were busy with at least two other traffic accidents in the same area. UHP troopers were so busy that Salt Lake police and Salt Lake County sheriff's crews were called in to assist.
Breur called Saturday's deadly day on I-80 strange.
"It's straight for miles it's bizarre," Breur said. "At first you'd think the sun was in their eyes, but the sun was on everyone's backs. It's just bizarre."
Earlier in the day, a 12-year-old boy was killed in a rollover accident.
The boy was in a vehicle traveling south on I-15 with his aunt and two cousins near Parowan at 1:40 p.m. when the vehicle veered off the right side, Breur said.
The driver overcorrected to the left, causing the vehicle to fall into the median, where it rolled two or three times, he said.
Breur said it was unknown whether the boy was ejected or was wearing his seat belt. The mother and two children, who live in Bountiful, suffered minor injuries.
Officials did not release the boy's name Saturday.
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