Authorities say a 19-year-old man dressed in a camouflage jacket and armed with a high-powered rifle gunned down his mother, step-father and grandmother at his family's home in eastern Tennessee.

Sullivan County Sheriff Wayne Anderson says six children witnessed the carnage Saturday evening near Bristol, Tennessee.

Anderson said Monday the suspect, Robert Seth Denton of Bristol, was packing about 100 rounds of ammunition during the surprise attack.

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

The sheriff says Denton's grandfather chased the suspect down a long driveway and shot him once in the arm. Anderson says the grandfather had been in a nearby trailer during the shootings. He says Denton walked up to the grandfather and told him he needed to call 911.