Outfitted in camouflage and armed with a high-powered rifle and 100 rounds of ammunition, Robert Seth Denton fatally shot his mother, stepfather and grandmother while six children witnessed the carnage in the family's double-wide mobile home in eastern Tennessee, a sheriff said Monday.

The 19-year-old suspect then went outside, laid down his weapon and camouflage jacket and walked to a neighboring trailer, where he told his grandfather he needed to call 911, Sullivan County Sheriff Wayne Anderson said.

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This photo provided by the Sullivan County Sheriff's Office shows Robert Seth Denton. Outfitted in camouflage and armed with a high-powered rifle and 100 rounds of ammunition, Denton fatally shot his mother, step-father and grandmother while six children witnessed the carnage in the family's double-wide mobile home in eastern Tennessee, a sheriff said Monday, Aug. 31, 2015.

The grandfather, Curtis Rose, had been making repairs to the trailer, and the din of power tools drowned out the gunfire. Rose went next door and found the horrific scene. He called 911, put the children in a bathroom, grabbed a gun and went after his grandson, the sheriff said.

Climbing into a van, Rose caught up to the fleeing Denton at the end of a long driveway.

Rose fired once, wounding his grandson in the arm, Anderson said. Denton didn't go far before calling 911 to report he had been shot and to give his location, the sheriff said. Denton was taken to a hospital for treatment and then taken to jail.

Denton was charged with three counts of first-degree murder, aggravated assault and felony reckless endangerment.

Denton made his initial court appearance Monday, and attorney Lynn Dougherty was appointed to represent him. Denton's bond was set at $1 million and his next court appearance was set for Sept. 8. Dougherty did not immediately return a call Monday evening seeking comment.

Authorities were still trying to piece together what caused the teenage suspect to allegedly turn on family members. Most of the children in the trailer lost parents.

"We have gained a lot of information that he has hated his mother for most of his life," Anderson said. "He had made statements ... over the years that he wanted to kill her." He didn't offer details about the cause of the enmity. The information was based on police interviews with relatives and others who know Denton, the sheriff said.

Killed were Denton's mother, Toshya Millhorn, 39; his stepfather, James Millhorn, 36; and his grandmother, Lena Rose, 57.

Authorities found Rose and Toshya Millhorn inside the home with gunshot wounds, the sheriff's office said. Both died at the scene. James Millhorn was found near the front door with gunshot wounds, and he died later at a hospital.

The children, ages 2 to 12, were in another room but witnessed the shootings, authorities said. Investigators have interviewed two of the children.

"These kids, they saw everything that went on," Anderson said, his voice shaking with emotion. "They've got to live with that for the rest of their lives, all six of them. It was the worst horrific scene that I've seen."

Five of the children are children or stepchildren of Toshya and James Millhorn, and they are staying with family members, authorities said. The other child was visiting.

One youngster, a 7-year-old girl, was injured when a fragment struck her hip, Anderson said. The suspect did not turn the gun on her, he said.

The sheriff has described the assault as a surprise attack. Denton parked his vehicle away from the residence and made the long walk up the hilly driveway to the home, Anderson said. Denton apparently entered the trailer through a back door.

A neighbor, Darrell Grey, said he heard a barrage of 10 to 12 gunshots and screams. The rampage lasted less than a minute, he said.

"We heard somebody, they said, 'Daddy, Daddy, Daddy,'" he said.

Authorities said Monday they don't know why the suspect spared his grandfather.

"Why he didn't shoot his grandfather and went out there and told him is beyond me," the sheriff said.

Investigators had not uncovered any prior local criminal record for Denton, the sheriff said.

Authorities said the suspect had served a short stint in the Army, but didn't provide any details about his work history outside the military.

As a child, Denton had lived in the trailer where the shootings occurred, but most recently lived in a Bristol apartment, the sheriff's office said.

No charges are pending against Curtis Rose for opening fire on his grandson, the sheriff said.