Authorities also said the bear, weighing as much as 300 pounds, probably was the same one who had been harassing other campers earlier in the weekend.
The dead boy's grandfather blamed federal foresters Tuesday for not warning about the earlier complaints.
"We're hoping that the Forest Service will do a better job protecting campers. It's been like a surreal nightmare," Eldon Ives told reporters at a news conference on his front lawn.
"The violent way he was taken is a sorrow that will never heal," Ives said.
Katie Baker of CBS station KUTV reports the first complaint about an "aggressive bear" was received Saturday night. Another was made Sunday, harassing another group of campers in the same spot before dawn Sunday. Kurt Francom said his son, Jake, was kicked in the head through a tent wall.
"It could have been my boy," said Francom, a school custodian.
"It hit me right in the face twice, and I raised my head up and it hit the side of the tent and smacked my head back down in," Jake Francom told KUTV.
Wildlife officers gave chase with more than two dozen dogs after the Sunday morning incident, but didn't catch the bear.
Sam Ives, the boy who died, his mother, stepfather and a 6-year-old brother were sleeping in a large tent Sunday night in American Fork Canyon, about 30 miles southeast of Salt Lake City, when the stepfather heard the boy scream "something's dragging me."
The boy and his sleeping bag were gone. The cut in the nylon tent was so clean, his family first believed the boy had been abducted, U.S Forest Service officers said.
Without a flashlight, the stepfather searched frantically for Sam and then drove a mile down a dirt road to a developed campground.
"He was pounding on my trailer door. He said somebody cut his tent and took his son," said John Sheely, host of the Timpooneke campground, who alerted authorities by driving down the canyon to a pay phone.