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Robert Seth Denton allegedly shot and killed 3 people and injured 2 others. (Getty Images)

A 19-year-old gunman shot and killed three relatives, and injured another in a surprise attack that was witnessed by several children in their Tennessee home. Authorities from the Sullivan County sheriff's office told reporters the attacker was shot and wounded by another family member as he made his escape.

Sheriff Wayne Anderson said six children were home during the shooting ambush, with one girl injured. According to Knox News, Anderson said the suspect, identified as Robert Seth Denton of Bristol, Tennessee, parked his vehicle near the family's home and opened fired when he reached the door.

"From what I've seen and heard, he just took 'em by surprise," Anderson told reporters. "It just all happened real fast." Anderson said two women were found shot in the kitchen and were pronounced dead at the scene. Another victim, a man found on the front porch with gunshot wounds, died in hospital.

"It's the most horrific thing I think I've seen," the sheriff said. The three victims were not yet identified. Denton, who is related to the victims, was charged with three counts of first-degree murder, aggravated assault and felony reckless endangerment.

The sheriff said the six children, including the child who suffered a non-life threatening injury, were in the living room when the shooting began. "There might be some shrapnel or something else that entered her left (leg) as a result of the shooting," he said. "He (Denton) didn't turn the gun on her. Something ricocheted or a fragment went into her leg."

According to the Associated Press, the child was taken to Bristol Regional Medical Center and is listed as being in stable condition.

Neighbour Darrell Grey told Knox News he heard a barrage of 10 to 12 gunshots and saw a man running down the home's driveway. "Everything happened so fast," he said. "It was just a barrage of 'pow, pow, pow.'" Anderson said another relative of the alleged shooter caught up to him and shot him in the left shoulder.

"Somehow this other family member got down to the bottom (of the driveway) and shot him in the car while the guy was driving away," he said. Denton allegedly drove a short distance before asking for emergency help. He was taken into custody around half a mile from the shooting, Knox News reported.